I came to about the same conclusion that you did as far as the list of power civs available. Here are some more detailed initial thoughts (not a ranking yet):
-Rome ... Very strong early game culture, leads to strong start and snowball potential. Weaknesses include (1) a target on the back due to early game lead, and (2) nothing to boost us down the home stretch other than the fact that entering the mid-game our snowball is a bit larger. Rome to me appears to drop off in strength as the game progresses. Mid-game conquest-leading-to-concession is probably the best victory condition to shoot for. The legion, however, is too early to take out a player, and as such I don't think it would be a good idea for early offensive warfare. By the time the mid-game rolls around, other players will have their UU entering the field. Rome really shined in the duel vs. TheArchduke, but in a larger field I'm inclined to see this as Fools Gold ... delivering ultimate disappointment in a long game.
-Germany ... slow starter with little to boost the early game, long term powerhouse. The combat bonus vs. city states can somewhat mitigate the lack of other early game bonuses. A quick conquest of a city state or two would definitely be the goal. The extra district ability is a stealth discount on nearly every district once we enter the mid-game, if we plan things carefully, due to the district discount mechanic ... I've found the limiting factor on getting the discount is often running out of district slots in cities. Being able to build the extra districts should enable us to get the 40% discount on nearly every district that we build. The extra military slot means that the more economically-oriented governments are more viable which is a boost to the mid-game, and a nerf to the religious line of civics/governments. Game plan: capture 1-2 city states ASAP, turtle up throughout the late opening / early mid-game, bash some heads late Renaissance / early Industrial
-Japan ... jack-of-all-trades, master-of-none. Not the strongest to a religion, but good chance at actually getting one. Early game somewhat slow, starts to hit stride in mid-game once district adjacency starts to build, and the adjacency-boosting policy cards are unlocked. The combat bonus on coastal tiles is mostly irrelevant on Pangea. I'm inclined to cross them off the list. The UU is less worthless now that anti-cavalry now qualifies for Agoge/etc. production boost. Hmmm ... this actually leads to an interesting opportunity. For Japan, Military Tactics is a single tech which unlocks combined-arms combo of anti-cav/melee. A beeline to Military Tactics (after Bronze Working, of course) includes the economic techs of Writing & Currency, and is a much shorter path than Machinery AND has all easily achievable eurekas. The issue remains though that Samurai can't be upgraded into and would thus require carefully set-up overflow and chopping to get out in an acceptable time frame. I'd still put this towards the bottom of my ranking
-Arabia ... Guaranteed religion, wouldn't get the best beliefs if not gunning for one (which would be long-term suicide) but with only 4 religions in play some powerfully beliefs will still be available. Russia and China are all but guaranteed to be in the game, and thus we'd only need on other person to go for a religion in order for the free prophet to pop. Since religion auto-spreads to all cities with a HS completed, even a late-founded religion can spread quickly. A nice religious ability though is VERY CHEAP worship building, and since the really good beliefs will be taken when founding a later religion the worship building is actually a decent first-choice. The faith generated from the worship building pays for itself in 12 turns, and some decent alternatives for culture/production/special bonuses are available. Mamluks come at the right time, but are not all that inspiring. At least they can be upgraded-into! Arabia is a slower starting civ, and the most important early yield to boost is culture. Culture unlocks the religious buildings and heads towards the Theocracy government. A nice (and under-appreciated) Arabian "ability" is that the 2nd level campus building is unlocked at Theology rather than Education! This means that early science can be lower-priority, and a couple of expensive mid-game techs can be ignored much longer. If Jesuit Education could be secured (unlikely) this is very powerful indeed.
-China ... Very strong early game if there is a suitable tile for Pyramids available with range of 1st/2nd city! Otherwise still a strong early game. Stonehenge a lock for an early religion #1 or #2. Various other ancient wonders easily grabbed on the cheap. Also suffers similar to Rome from early game target-on-the-back-syndrome and I-want-to-steal-your-wonders-syndrome. The additional builder charges are a late-opening / mid-game buff saving many hammers and build queues for other items. UU is almost irrelevant with only 1 range. Sure it might be decent on defense, but who wants to defend? And it can't be upgraded into. China looks like an exercise in efficiency to make the best use of the extra builder charges and squeeze the most out of early wonders to build a snowball that can out-roll later-blooming civs. Beware of renaissance/industrial invasion!! ... I'm inclined to pass
-Gorgo ... Strong early game due to culture farming barbs and extra wildcard slot (either extra economy card, or GProphet/GGeneral card). Hoplite is now boosted by Agoge! Should get out of the starting gate close to Rome, but I like the longer-term prospects much better. An extra wildcard slot in the governments unlocked at PP along with the cards that start getting unlocked gives great flexibility and can adapt better to in-game needs. The Acropolis does eat a hill, but apparently provides 2

from city center adjacency and 1

from all other district adjacency, which is far superior to the district it replaces (the extra envoy can open up some interesting options, although in the grand scheme it will only add maybe 3-4 total ... how many Acropolis does one really need?). Hoplite is buffed from boosting with Agoge, too early for aiding in conquest but mildly useful for discouraging Horseman attacks in the Classical era. I think the wildcard + culture-farming make this is a solid all-around choice.
-Russia ... Guaranteed religion, maybe not the 1st if China is in the game and rushes Stonehenge, but almost guaranteed to at least get the 2nd religion. Early religion means powerful beliefs can be secured, however early also means that the beliefs won't have much immediate impact. Jesuit Education doesn't have any impact until Campuses are built and culture accumulates. Then it is a long time until Theaters are unlocked. Choral Music requires building Shrines which is a drag on early expansion. Extra starting tiles is awesome and provides much greater flexibility in locating cities and acquiring resources. It also can boost the early game through quicker access to better tiles in the 2nd ring. The strength of the Cossack is also a weakness through eeek-cripple-Russia-before-Industrial syndrome. The trade route ability is mostly useless. Extra faith and production in tundra can provide marginal situational benefit. If the capital has a tundra resource in 2nd ring that gets acquired upon founding (which happens in randomly generated maps that take starting bias into account) it can make early pantheon easy without running God King. Overall the easy access to religion + Cossack make this a solid choice. However since you are currently playing Russia in PBEM 7 I assume you'd prefer passing on them here

no complaint from me if we pass on this one.