I haven't really had a problem with corruption...yet. Though my income is about 25k denarii at the moment.
Oh, don't worry, I'm well aware of the directional nature of shields, and the Hypaspistai always get attacked from behind.
I usually don't have much in the way of Cavalry (I'm a cheap bastard), but I usually make sure to get some troops attacking at least the side flanks, if not the rear. It's necessary against things like phalanxes. You use one legionary group to keep their phalanxes pointed in one direction. If they are using the sarissa, it's really hard to close the distance and attack, but if I can get some guys in on the side, I beat them up with sword to sword combat.
Though it is mighty fun blasting through the flanks of a group in combat with cavalry and watching bodies fly. It typically use my General's unit though, as they have more HP and when they die, I'll eventually get free replacements anyways.
Which Sarmatian Cavalry were you talking about? I was able to buy some Sarmatian Archer Cavalry, and they kick ass!
If found the Framea Germani units fine as well. The large stack is nice, and I like melee units that have the few missile weapons before charging. I was just so used to my legionaries that I was surprised how quickly the Germani units were getting killed relatively speaking. More a compliment to the awesomeness of the legionaries