The only mount that is actually required for a weapon is the APWM (with armored gloves counting as one "free" APWM), regular BA weapons only need a specialized mount if they are to be easily replaceable (MWM, configurable turret) and/or reduced weight (SSWM, DWP). Detachable missile mount is, AFAIK, technically just a MWM mounted on the body. I can't swear to that, thought - I've read different interpretations. :-\
To make an extreme example:
IS medium BA, mounting:
fixed LMG (75 kg, 1 slot)
MWM with LRR (175 kg, 2 slots + 10 kg, 1 slot for MWM)
SSWM with OS-LRM2 (40 kg (half of 80), 3 slots + 1 slot for SSWM)
APWM with assault rifle (5 kg, 1 slot, weapon free)
This means a squad of 4 has:
4xLMG
4xLRR
4xAssault rifle
1xLRM2(OS)