I practice Religio Romana! I can try to answer any question you or anyone else has.
The Romans were fine with other gods being worshiped in the Roman Empire as long as the Roman gods were also worshiped and placed first. For the Romans, religion and the state/empire were very closely linked together so honoring the gods also ment honoring the state/empire. There have been cases were the Romans adopted non-Roman gods too. Isis is a great example of this. A cult of the Egyptian goddess Isis was formed in Rome and became pretty popular. Epona is a Celtic-Germanic goddess who was also worshiped by some Romans. Images of her were placed at stables due to her connection with horses. The relationship between pagan Rome and Christianity however did not coexist as nicely.
Hellenism is more popular so you will find more people who follow that religion than Religio Romana.
Here are some links to check out:
Cultus Deorum (active facebook group)
Nova Roma (old site)
Nova Roma (new site)