Frontal charges should be avoided with Cats as well as with any cav unit. Cats can get away with it against lighter infantry, but still try to hit them when they are not braced for the charge. You are right, unless a Cat unit is upgraded quite a bit, prolonged melee combat can result in heavy casualties, especially against heavy inf. If you leave them much longer after the charge bonus is negated, then they become basically a mounted infantry unit, without advantage of numbers or formation.

If you can get any kind of success out of Round Shield cav, you should be happy.

Poeni infantry is unique to Carthage. Egyptian inf. units are Nubian Spearmen, Nile Spearmen, Desert Axemen, Pharaoh's Guard. I don't think I'm missing any, please let me know if I am. The two spearmen units are basically worthless, with PG being one of Egypt's unrealistic units, as Reluctant Samurai already pointed out. I think I started a campaign with Egypt, but quit after two turns.