Welcome to the .Org and to EB

The Italic general represents the "native" Italic peoples, not the Greek and Celtic colonists. Since Patavium is thoroughly Celtic, you can only recruit the Celtic general over there. It's possible to make the Italic general available in Patavium, but it's difficult to work out how to do this in just one province. Such questions are better asked here, anyway.

I am not sure what you mean with your second question. You cannot build factional barracks (that recruit Roman troops) outside of your homelands* until the Polybian reform, and not outside Italy until the Marian reform. This is how the game is designed.

* The Roman homelands are their starting provinces plus Segesta. Patavium is a type II (Expansion) province, so you must wait until the Polybian reform before you can build Factional barracks and recruit Roman infantry.