I can find no legitimate material that explains how orbs work when creating a custom wizard, and choosing Cosmos. I believe that by putting certain orbs into Life, Earth, etc. it allows you to used higher level spells of that school. Maybe 1 orb per level? I am unsure. However, since the highest spell level is 4, or so I believe, this would mean that you can access the highest level spells by putting 4 orbs into something, then having more leftover for other schools. That would make all other spell school choices other than cosmos irrelevant.