As different atoms in geom mean different basic functions and different number of basis functions if you use basis sets from NWChem default libraries. That's the reason why you can't use the bare cluster as an initial guess for H2 one. There must be some way to keep the basis functions unchanged to do it.
But when you use a system with triplet O2 as an initial guess for a system with quintet O2 , I think it's OK.