Ok, I'm 99% certain you're neon tetras are NOT pregnant. With everything else that's going on in your tank it's more likely they're suffering from an internal bacterial infection.
We need water stats, any chance of pictures? Complete run down of inhabitants and symptoms. It wouldn't hurt to do a water change today and tomorrow, dilute whatever is causing the problems.
As for your rtbs and rainbow, these 2 fish are not compatible, if you say your rtbs bullies the rainbow, it's only a matter of time before there'll be a murder. I would suggest you rehome 1 of them immediately. Also, your rtbs hiding is totally normal behaviour. RTBS's are very shy and territorial, they like to have they're own secret place where no one can see them or get to them, if they don't get this, that's when they become aggressive. However a rainbow in teh same tank is like a red rag to a bull.