IMO, you should match tires on an axle if at all possible (except in an emergency). You should match inflation for sure (even though it is capable of 65PSI you don't need that much air for your load). In fact if you check your spare you may find your rim is only capable of 50 PSI in any case.
The extra plys of the D rated tires are a good thing. Just put an identical one on the other side of the axle. Keep the C rated "good tire" as a spare.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW