First of all, thank you very much for taking the time to post, I really do appreciate it!
While it is difficult to assess which of ClamXav or Virex will work best for you, I do not think that both applications would interact with each other in a damaging fashion as long as any automatic or background scanning feature is turned off.
Regarding the Clamav user account, I do not think that it should be the cause of any concern either as it will, unless used, dutifully sleep on your machine. Mark Allan is aware of the existence of the account and there was talk on the ClamXav forums to re-use it as part of the installation process of the application.
I hope this answers your question and, needless to say, remain at your entire disposition to provide you with any additional information you may deem useful.