Can you transfer from an RER train to a regular metro train on the same ticket?
Yes. But not always the reverse. It depends on how far out you're going.
Single t+ tickets are good anywhere the metro goes (some lines end in zone 3) but only in zone 1 for the RER. So transfers from RER to metro or vice versa are fine within zone 1 with such a ticket.
Yes as the two previous posters are correct. One thing to mention, depending upon the RER exit you may need to get through the turnstyle with your RER ticket, so hang onto it just in case.