Unless specifically stated either by scenario SSR or by a CG/pack specific general rule then there should be nothing preventing HS combining. That is regardless of whether a scenario card has enough HS to man your MMG/HMG. From memory HoB had a generalised rule that only 127/228 crews could man certain SW (including MMG/HMG) in some of their packs but absent that no limitations on recombination.
In recent years I have seen one or maybe two scenarios where some SW could only be manned by the supplied HS, fair enough, but if you were unlucky to X out a pair of SW there was nothing preventing you from combining the now unemployed HS, as best as I can remember.
In recent years I have seen one or maybe two scenarios where some SW could only be manned by the supplied HS, fair enough, but if you were unlucky to X out a pair of SW there was nothing preventing you from combining the now unemployed HS, as best as I can remember.