I've played HARP using both the rules in the core rulebook for combat, as well as those presented in Martial Law. I felt neither gave a good result but Martial Law was better. I've played Rolemaster and enjoyed it and I've heard that it is possible to incorporate the combat rules from Rolemaster (Arms Law) straight into HARP. Is this possible? Has anyone done this and what are the effects on play balance? (eg do encounters need to be tweaked, does it affect other aspects of the game other than combat?)
HARP and Rolemaster are scaled the same way for hits, skills, and character levels; it should be no issue at all. The HARP combat system looks very much like one of the options in a supplement for one of the editions of RM that I have.
I know that MERP, also, is quite capable of such substitutions, but most people I knew were going the other way... MERP combat in Rolemaster.