That's a difficult one. Most alliances Rome had would be "client kingdoms" and similar at her borders, but these are eleutheroi in EB1 terms and don't do diplomacy. Of course: the dividing line is vague, since IIRC the Aedui were also such an ally at some point.

Anyway, R:TW's diplomacy A.I. is stupid. Alliances mean very little to it. A better way of ensuring a good relationship is a small regular tribute (say 200 mnai per turn). In-game, an alliance does little except that you cannot be allied to two factions that are at war with each other (although for some reason this does occur at the start of EB's campaign).

I guess you should ally with a faction if their well-being is important to you and you do not want them hostile. However, don't trust the A.I. It has been programmed to dislike the player, so even your allies can quickly turn against you.