I have a suggestion for Persia.
Rome should not get "free" units that automatically spawn in Persian territory. This is for two reasons:
1. Historically inaccurate. While Rome and Persia/Parthia (whatever dynasty) certainly were at war, the battles were generally indecisive with neither side gaining much ground. In other words Persia never got anywhere close to Rome, nor the other way around. My understanding is almost all of their battles took place is the eastern part of modern-day Turkey.
2. Makes playing as Persia too hard. It's bad enough when you have to deal with the Greeks already, but right after that going with rome as well? PLUS you are somehow supposed to conquer so much territory? Yeah right.
Personally I think Rome (regardless of whether being controlled of A.I. or human) should be given "free" units perhaps in Consantinople (supposing they capture it. If they can't manage to capture it they suck and don't deserve the bonus). With say, four free Pratoreians they could march eastward until they more or less meet a stalemate with Persia, as they really did. It would still give Persia a challenge but not annoying and stupid Prateorians magically popping up out of absolutely nowhere deep in Persian territory.
I also think a better UA for Persia would be it gets to build the UU in enemy cities.
For example if Persia captures a Babylonian city they could build bowmen, if they capture an egyptian city they can build war chariots, and so forth. This would incorporate how the Persian empire took from the cultures they conquered... perhaps on that same line they could get the UB's as well. Needless to say, though, Persia themselves would have to have the technologies researched in order to build that UU/UB even if they happen to have captured the city.