I understand your concern and I would feel the same way if I were adopting a cat from that room. Maybe the FIV+ cats have been vaccinated against FeLV, so they aren't at risk, although my understanding is that the vaccine doesn't produce total protection in all cats. I would ask that question before you bring home Laslo (he's really cute!). With their weakened immune systems, FIV+ cats need protection. I would also worry about the FIV+ cats infecting the FeLV+ cats.

I hope all works out for you and Laslo.