I just started a new Gatai campaign today and haven't had any issues with money, in fact I didn't disband any troops and have been able to build something in every city, every turn. I took the faction heir and his army to Kallatis, and when I beseiged it the rebels sallied forth and were crushed. The faction leader took the rest of the soldiers north, beat the rebel army and then beseiged Sarmiszegethusa, taking it immediately as my spy had opened the gate. My financial situation was considerably improved by the fact Makedonia gave me 9000 for a ceasefire.
I intended to sit back and consolidate for a while as I try to keep a one city per family member empire and don't accept adoptions so had to wait a few years for a son to come of age. My plans had to be shelved when an Epirote city rebelled to me kicking off a war with them. It's now 256BC, I only have to take Nikaia to "unite the tribes" and I have only just raised another army after only using the opening troops to take my current possessions.
In terms of battle tactics, when attacking settlements you could seige them and wait until they sally forth. Few of the rebel settlements around you have armoured troops so your archers will get shedloads of kills while they are milling about getting into postion. The light phalanx can hold an enemy in place while you send your falxmen or Costobocii Axemen round the back to slaughter them. Your FMs are good for that too, just remember to alt-right click attack after the initial charge to draw their swords. I have been annhilating the Epirote armies attacking me and I haven't even built my MICs up to allow me Rhomphaias yet, nor have I been using any horse archers so they are far from difficult to use in battle, in fact I would even say they are too easy to use.