We had said that there was no requirement on the astromech... but I do think it would be cool maybe if we made it a rule that you couldn't use a regular dome on him. But that's up to everyone else; not me. For example though, you can come up with all kinds of different astromech head designs... I made these a while back using the bar stools from the cantina sets -

For the protocol droid, the reason I had suggested not using any existing 3PO parts was to get some variety/creativity going instead of a basic C3PO repaint...
But again, I'm not the rule maker here. Majority rules. Whatever everyone wants to do is fine.
