Would you agree that, although there is a finite about of matter and space in world-systems, there is an infinite amount of matter and space outside of (ie between) world-systems?
Yep. I'd agree with that. There are other cosmoi out beyond our world-system taking up some of that infinite matter, then probably some matter floating around between world-systems.
But, by definition, the intermundia/metakosmos doesn't have a world. If it did, it would be a cosmos/mundus.