Lionfish are generally peaceful so keeping them with other fish isn't a problem. They're venomous, yes, but that's more of an issue for us humans if we were to handle them!
The larger the tank the better. I would recommend no smaller than 40 gallon for the smaller lionfish. They're not really active fish but they do need room to swim and they require very clean water which is easier to maintain in a larger tank. They also need hiding places so they don't become stressed.
Yeah like saratang has said, lionfish aren't really the territorial sort (in fact, they can be shy). They can get along with most fish but they have fairly large mouths and are carnivores so don't put in any fish that can fit in their mouths.
And being venomous you need to be very careful whenever you have your hand in the aquarium.
I'm new to fish keeping but this is what I've learned so far! Fascinating fish for sure! Although I don't really know if they're good beginner fish.