Well generally you don't need more than 2-4 units to take any towns because the AI doesn't reinforce them very well. And of course you can just maintain a siege until an attacking army turns your siege into a field battle. If you need to storm town centers use cheap expendable units with high lethality or ap. Gallic swords are perfect for this.

As far as field battles, phalangites generally take very few casualties and don't need to be replaced often. Heavy cavalry I keep in reserve. Light cavalry gets the brunt of the work along with missile units and the mercenaries or local recruits. Since I don't care about replenishing mercs I use them as liberally as I'd like. Light cav will get beat up but cavalry moves much quicker on the strat map than infantry reinforcements. And of course, missiles aren't that big a deal as they shouldn't take many casualties either, especially in the West. Therefore, light cavalry are really the only reinforcements I need regularly and I try to use mostly Prodromoi as AS. Also try to avoid attacking enemy armies, let them come to you and only push the issue by sieging cities. That way the enemy tires themselves out coming to you and allow themselves to get beat up by archers and slingers. Recruit Rhodian Slingers and Bosporan or Kretan Archers if you can.