My simple rule of thumb for installing Romani governments is:

Always install Type IV first, and then upgrade it when the client ruler dies.

Unless:

1. The town is very small. If it's not even a large town, then it's not important enough for Rome to bother pandering to the local population by letting them have a client ruler. Install a Type III first.

2. The town was the largest settlement, or original capital, of an enemy faction. Letting them have a client ruler will make them think their subjection is only temporary, and encourage them to rise against Rome in the future. So they can't have a client ruler. Type III government first, or even Type II if they were a particularly dangerous enemy faction.

3. The town was enslaved or exterminated when you conquered it. You can't put a client ruler in after doing that - the people hate you and can't be trusted. Type III first.