Not sure how helpful this is, but I'm playing the Macedonians, so I'm in a similiar situation to you.

I'm assaulting the Italian peninsula with a three pronged attack in consort with the Gauls.

Like you, I started by attacking Croton. At the same time I sent a full stack to attack Apollonium, and had my Greek cities build up more units. Once Apollonium was taken, I garrisoned it with the mercenaries I had been using to guard the mountain passes between my territory and the Brutii. I then shipped over the conquest army to attack Tarentum.

I managed to take Tarentum, and also built another army stack from the units I'd been making in Greece. With the new army, I marched up the Croatian coast taking Salona, Segestica and Patavium, and so wiped out the Brutii. I had to use mercenaries quite a lot to keep this army up to strength. This was real blitzkrieg stuff - I brought a ballista with me so I could attack the cities on the same turn I began the siege.

Meanwhile I sent the Croton army to attack Capua, and split the army in Tarentum to make a new garrison for Croton. I also sent a diplomat to make an alliance with the mighty Gauls, who are attacking the Julii and keeping them occupied (the 'third prong' of my attack).

I've currently got Rome under siege with the army from Capua (I've tried to assault the city twice, and been beaten back, so I'm just going to starve them out), and my army from Patavium is attacking Ariminum although they are getting attacked a lot by big Julian armies.

Things aren't going all my way, because the Romans have sunk half my fleet, and the Scipii are attacking Croton with massive armies (losing their capital doesn't seem to have stopped them), but they are definately on the defensive. I'm thinking of making some more armies in Greece and sending them to attack Sicily to try and divert the Scipii, but I need to build a bigger fleet first.