If I currently have Verizon FIOS and record using WMC + CableCard on a Win 7 PC + Ceton InfiniTV 4 tuner
That should all be fine as long as you are not recording Copy Once programs using your CableCard. Copy Freely programs, or programs with no DRM, are already supported on JRiver MC. For Copy Once, see
http://yabb.jriver.com/interact/index.php?topic=101468.0If I install the MC TV recorder add on - will it cause conflicts with the existing WMC setup? or do I need to uninstall WMC first?
You will need to stop WMC from trying to use the TV tuners if you are testing or running MC TV capabilities. You don't need to uninstall WMC, but you need to stop it, and its two services, from starting with Windows. There is also a setting in MC that disables WMC to avoid conflict over Infrared commands, at Tools/Options/Theatre View/Behaviour.
Likewise, if for whatever reason the JRiver MC21 TV doesn't work out, can I just simply run WMC and it'll run the way it always did?
Simply turn off the MC TV feature again in Tools/Options/General/Features, turn off the disable WMC setting at Tools/Options/Theatre View/Behaviour, and re-enable WMC start-up with Windows. Of course WMC will need to catch up with EPG updates and so on, but should work as before.
There has been quite a bit of discussion around WMC behaviour vs MC, particularly in the Television forum. Do some searching and reading to learn more about any the differences and what to expect when converting over to MC.