S.K.
My two kids share a room and have since my son was 3 months old (they are 21 months apart). My only solution has been to put my son to bed first in the crib so that he can get to sleep before his sister comes in. My daughter is like your son, she will not always fall asleep right at bed time. My solution has been that she can quietly look at books or quietly play with her dolls, but she MUST stay in her bed. I really haven't had too many problems with her messing with her brother (maybe 1 or 2 times). As for them waking each other up, it will probably happen for a couple of weeks, but they will eventually get used to each others "sounds" and sleep through each others cries in the night. My daughter also wakes up crying out often through the night and my son sleeps right through it, and when my son was still nursing at night my daughter slept through his cries. I think it also helps that I put some soft bedtime music on for the kids when they go to bed, it eventually becomes background noise that they hear in their sleep and I think it kind of masks the noise they each make. I will tell you that the first few weeks is going to be very rough (and possibly longer depending on your children) but it will get better eventually.
Also, I would get them sharing a room as soon as possible because the sooner they start sharing the sooner you can get the "bad" nights over with :). Especially if you plan on starting your son in pre school any time soon.