Hi,

the romans are definitely op in vanilla. Due to the necessity of the civil war in order to win the game you will have some fun battles in the late game.
As for the other factions, they are all somewhat unique and require a different approach to strategy and battles. Just give them a try, it's interesting.

As for the mods: Practically every mod will slow down the campaign pace and will try to balance the game a bit better. Most of 'em have a larger map. Europa Barbarorum is slow paced, history oriented and very educating. Fewer and therefore decisive battles. In RTR you will have more battles, and I personally like their skins. I wouldn't say this one or that one is better. Each mod takes a different approach. Cannot tell you about Roma Surrectum or Fourth Age, though. You might want to go over to twcenter.net, there are some more nice mods in the mod section there.

Oh and the elephants, I personally find them overrated, but they can be the decisive factor in a battle, if used correctly. I tend to keep them back until the enemy is pinned by my battle line, then have them coming in from one flank, stomp through to the other flank, retreat, rest (resting is important!).
They become extremely vulnerable in the presence of archers (fire), many skirmishers (using eles against numidia is quite pointless imo) or artillery.