If you're connected to a SQL DB then it can handle up to 50k clients. As long as clients can talk back to SEPM by opening any needed firewall ports then it really shouldn't be a problem. Being on a different domain doesn't matter to SEP/SEPM communication:
http://www.symantec.com/docs/HOWTO81103
Yea, you can use the same installer to install the same version. The installer is universal so that's fine. You need to make sure you have the correct amount of licenses before adding clients so you're not over-deployed.