There’s nothing wrong with installing Bitdefender or Malwarebytes in an MSFS computer.
Just disable real-time scanning and use it for system scans.
You can generally create exceptions (programs the software won’t scan in real-time, but to me that’s more trouble than it’s worth - especially if MS Defender is your only real-time scanner.
Different anti-malware platforms have different definition lists, and use different heuristic detection algorithms. It’s never a bad idea to use more than one for full system scans.