1: Set very high tax rate in every single settlement you capture, and leave a family member to garrison it if need be. Build Markets and Ports and Mines as soon as possible. Ports being priority, then markets. Mines come last because they're the most expensive.

2: Sweboz tend to go eastwards. You don't really have to worry about them. After consolidating Gaul you are practically free from threats. You can choose to invade wherever you want. Crush the Romani before they hit you. Or take out the Sweboz. Up to you. In my current campaign I beat up the Romani, then turned on the Sweboz.