1) Have you done a reinstall of your OS? It will take care of 99.99% of viruses I know of. After the install, scan all your backup files before reinstalling.
2) You can reuse your RAM as long as supported with new system.
3) Yes, you can mix as long as they are able to run at the timings, voltage & MHz of the slowest RAM.
No i didnt reinstall my os because i couldnt find the backup but i was about to replace it anyways because it was slow so i just want to salvage what i can and throw out what i cant.