the "capellans don't have a monopoly on heat weapons" is apt..
not only has the Plasma Rifle spread thru out the IS (the cappie just use it more often than other nations), but Flamers and Inferno's are both widespread and common in both the IS and clans. Inferno's especially, since they are ideal vehicle and BA killers.. something you can expect to face when fighting Cappie's due to their augmented Lances.
the clans have Inferno's and Flamers too, also the Plasma Cannon.. a weapon that is superior to the Plasma Rifle in imparting heat with twice the heat transfer, and is half the mass too so effectively is 4x better than the Capellan weapon. and for the clans they are common, and used quite often. usually in clusters of 2-3 at a time.
and lets look at the Capellan's main enemies? the Davions, long time extensive users of Flamers and Inferno's, now buying and producing clantech like mad. and the ROTS, with widespread use of flamers and inferno's, also clantech out the wazoo and entire clan enclaves.
not to mention, there is the issue of capellan strategies.. quite often we hear of them setting entire woods afire, or torching a town.. Capellans are happy little pyromaniacs now, and because they tend to do this before fighting in said flames to route an enemy, one can understand why thery'd come up with a way to keep their usually heat sensitive mechs cool as they wade through the flames cackling...