Well, when playing Romani I buy only the following mercenaries:
1. Kretan Archers on the South - for obvious reasons; even my Regional MIC Hellenic Archers are not as good... and I spend my money on constructing other buildings than Regional MICs, at least at some stage; nevertheless I use Hellenic Archers in "homeland defence" against some unexpected invasions or rebel armies.
2. Gallic Archers on the North - it's much better to have them than not to have... and I still have lots of other buildings to be built than regional MICs. I'm developing one large Gallic Auxiliary Center in Mediolanum, but this will take a moment... and I need archers to counter those ugly Gallic/Spanish/Lusotannan skirmishers :D
3. I have once bought Saka Horse Archers in Thrakia - yep, they're quite nasty and fast learning boys... and can also engage from distance those bad light cavalrymen of Hellenic states that try to harass me with their javelins...

No other mercs I ever buy - maybe until I expand so to get some Saka/Sauromatae armored cavalry (cataphracts or similar), yet it wiill certainly take some time.

BTW: I think that it's pretty easy to get Roman Polybian reforms quite early in game - in 262 BC I conquered enough in Gaul and Sicily to get reforms... and so It's wise to expand in at least one city the Camillan Barracks to max. level to be able to upgrade Camillan units. The Polybian reforms start only when Camillans get their first chevrons and it would be a waste if I have to disband them. Some of them can be used in homeland security :P , but some of them can campaign alongside (or alone) abroad, gaining more experience.
I only realised that Samnite Heavy Infantry is unaccessible in Polybian Barracks, so it's better not recruit them at all or build max level Camillan Barracks in Arpi and keep it on.

I myself still have Camillan Barracks in Arretium (level 4, only... :( ), Arpi and Ariminum (level 3), while Polybians level 5 are in Roma and Capua, lvl 5 currently being built in Taras, lvl 2 in Segesta & Patavium and lvl 3 in Bononia (currently being under upgrade to lvl 4).
Thus my Camillans combat in the North, Gauls and Spain, while Polybians can fight in the South: garrison Sicily and fight in Illyria and Greece.