The Iberian infantry are only really useful if you mod their stats a bit, they are just too weak to do anything otherwise. I gave them 3 shield for the caetra, and 4 armour. Their animation in game is wearing nearly as much armour and helmet as hastati...and certianly more than the barbarians wearing only pants. Yet the Iberians get 2 armour, various barbarians wearing only pants get a 3, and hastati get a 5. The small shields should be worth about a 3 in combat. I am OK with their offensive stats, although at the time the Iberians actually had better swords than the Romans.

After Iberians you only get Libyan spearmen. While they are not a bad unit, they are weak on offense. It is not until you get Poeni that you have a real first line infantry unit. With Carthage, focus on building stables and cav heavy armies, your infantry is not going to win you battles early on. The round shield cav don't have great stats, but they have good mobility. The longshield cav are pretty good. Learn to hammer end units with 2 or 3 cav units at time so that they melt away. This will rapidly develop into a chain rout if you maintain pressure.

Buy Balearic slingers whenever available, and use merc hoplites to provide some backbone while you are waiting for Poeni to become available.