I try to approach composition not so much from a view of forcing the historical composition into the game engine constraints, but from achieving historically important tactical goals within the limitations imposed by the engine. I want cavalry to screen the flanks and charge pinned enemies, so I keep 2 equites. When I can afford it, I want 2 jav cav in the form of Campanians to harass before contact, melee, and pursue routers (especially routed family member heavy cav, which they are faster than). I keep 2 triarii, 3 principes, and 4 hastati in a pyramid version of the triplex acies, as that preserves most of the tactical benefit of the 4-4-3 historical system for a 4 unit front while reducing the number of units from 11 to 9. In the Camillan era, I use 2 units of rorarii either on the flanks of the principes as a flanking cavalry sponge, or in a thin line in front of the hastati as a screen to pin infantry in place for the hail of pila. 2 units of leves and one of slingers deploy in front set to skirmish, and I keep a unit of archers in the back between the triarii and the general behind them. In Polybian, I'll use hastatii Samnitici in place of the rorarii and allied slingers in place of the accensi.
For an allied army, I'll use 4 medium infantry staggered up front, two heavy infantry and a pedites extraordinari as my second line, 2 heavies in reserveand a pair of spearmen on the second line flanks, with cav, skirmishers, slinger and archer per the above, and either a family member or an equites extraordinarii as general.
Haven't made it to the Marians yet, but am thinking 4 cohorts up front, 2 plus a first cohort in the second line, 2 cohorts in the 3rd as reserve, and an auxilliary spearman on each 2nd line flank with skirmisher and cav support.