Well it's a requirement to disable your plugin if it's not loaded by L4D/L4D2 so please do that anyway

.
Team Versus does use the mp_gamemode 'teamversus'. It is completely the same (as far as I know) as versus except team switching is disabled. So yes you will need to detect that gamemode properly or your plugin will be not working correctly. A lot of people play Team Versus then force server so it goes to their server.