What I am doing in mz ongoing Roman campaign (currently cca 220 BC) is to roleplay my stacks as "legions" composed of two "alae".

The first "ala" is the Roman core, which is uniform for all my legions and is composed of:
1 velites
2 hastati
2 principes
1 triarii
+ there is the legion commander, FM of at least Questor rank and a second FM (mostly commands, rarely engages in actual combat), usualy a military tribune to gain some military experience, to serve as regular cavalry and to command the "allied ala" in minor battles.

The "second ala" represents the Roman alies and is regionaly based. For instance:
The "First Legion" (Legio I Italica) is complemented by Italian socii:
1 Aichmetai Leukanoi
2 Pezoi Brettioi
2 Hastati Samnitici
1 Equites Campanici

There is a force raised and maintained by allied greek poleis of Magna Grecia (Type IV gov in Taras and Rhegion) which gives another "allied ala" to complement legions operating in the vicinity of Southern Italy:
1 Toxotai
2 Hoplitai
2 Peltastai
1 Hippeis Tarentinoi

Or there is a force raised and maintained by allied Celtic communities of Liguria (Type IV gov in Segesta) to complement legions guarding the Northern Italy:
2 Sotaroas (Celtic archers)
3 Gaemilae Liguriae
1 Ligurie Epos (Ligurian light cavalry)

Other regional "alae" are composed along similar lines in Emporiaon, Massilia, Syracusae, Sardinia, Northern Dalmatia, etc. In times of peac they form garrison in the provincial capital and represent the army which the idependent allies raise for their protection and are commanded by their own king (cient ruler). In emergency, "roman ala" is sent to the province, merge with this "allied ala" to form a full-strenth legion under Roman commander and set out on campaign.

In total the legion is never more than 14 units (2 FMs, 6 Roman, 6 allied) which makes the battles more interesting and even allow the retarded AI to inflict occasional setbacks on me.

According to my house rule I have Type I gov installed in Italia proper only (Rome, Capua, Arpi, Arretioum, Ariminum), Type III in Messana and on Corsica and Sardinia. There provinces pay upkeek of the Roman core units of my legions and of my navy. The rest of my provinces are all Type IV and I make sure that the total upkeep of regional units payed by each of those provinces never exceeds the combined tax and trade income of that particular province.

I think that this allows for quite a historical gameplay, giving the allies some degree of independence as well as for fun and enjoyment, because the varibality thus achieved prevents the battles from becoming repetitive and boring.

I am very gratefull to EB that it is so well designed for role-playing. The possibilities are countless!!!

P.S.- Another house rule of mine is to auto-calc all battles where the AI attacks my besieged cities, because the AI sucks so much in sieges that it is almost unfair. This way it at least have some chance.