The RV-12 tail and wing kits are compatible with the RV-12iS fuselage and finish kits. They are actually the same kits. There have been a few relatively smaller changes to the tail and wing kits over the years, so depending on when your parts were originally made there may be some minor differences, but that should not affect ability to use for a 12iS.
Our tech support team can, of course, answer specific questions.