There is nothing to stop you using more than one on-demand anti-spy/malware application. The only real restriction is the number of resident on-access anti-spy/malware (AS) applications and that should be restricted to one.
Spyware Terminator is the only free AS application with resident protection, if you use this don't install the toolbar or crawler or the anti-virus module). Some elements of the programs might not work if you have an older OS like win9x or winME, this is namely the resident protection in SpywareTerminator.
The other two you have to pay for resident protection in the Pro versions.
See my signature.