That's a great quote and I think that's consistent with Dewitt's view too that Velleius (taking that Cicero is simply following some contemporary Epicurean text) has it right.
But just like Velleius doesn't stop there, and he joins with it the isonomy argument, I am thinking that there is no reason that *we* should stop there either. We should also factor in the Epicurean argument for life on other worlds, existing in "equitable distribution" (based on how we see a progression of life here on earth). From those we can conclude that living beings more blessed than us, and deathless, are as certain to exist somewhere as are the atoms, which are also beyond our sensory reach, but about which we are confident, based on reasoning from observations that *are* within our sensory reach.
As far as the gods living "between" the worlds, that sounds to me more like one of those manifold possibilities that must satisfy us, rather than a requirement that they live *only* in that location. The requirement would be that they have mastery over their environment so that it provides all that they need for their happiness, and we could suggest numbers of possibilities for how that could be set up, rather than thinking that "intermundia" gives us a complete physical description of a specific particular location. The main thing is that we see no evidence or reason to believe that they would take any notice of us, wherever they are and no matter how many of them there may be.