Either both Scribe Girl and Cullen will be straight or both will be bi. That's what I think.
There's not justification to make them one or the other since sexuality has no bearing on character, so I can't even begin to guess which it will be.
I also think we will have the 2/2/2 romance split.
The only thing that makes me question my thoughts on the NPC romances is the revelation of Cass and Cullen as LI at the same time. It strikes me as weird to release one straight and one bi companion as the first two LI. So both Cass and Cullen could be straight or could be bi. Seems bi would be the likely candidate since both are very popular. But how odd to announce a companion and NPC as the first two LIs.
I don't think there will be class restrictions, but I think there could be race restrictions on account of how difficult it would be to make a qunari and a dwarf interact physically (even just kissing). Then again, that's also why I think Varric and IB won't be romances.