1. Primary orbital slot and slot 0 around a body count as different orbits(99% sure). You should be good to build there. (EDIT: Apparently ports around a body divide economy distribution and market supply/demand between themselves, so that coriolis might not be as well-stocked as it should. This could possibly be remedied by generally improving the system, thus boosting the market size. Speculating there.)
2. We just don't know yet. I've seen somebody place a rocky asteroid port around the innermost slot, only for it to end up in the icy outer ring. Might be safest to just wait until that mechanic is clearer/less buggy.