For the Panzersoldaten i'd probably just do one modular mount in the body and leave the arms open for anti-personnel mounts- for some reason I like the idea of keeping the Squad support mount exclusive to the Dracs and having the Lyrans focus on large missile launchers.
All of them are designed as optimized as possible after the primary flavor has been achieved. I abhor the whole idea of "you must design units badly to give them flavor" - I will always optimize as far as possible given whatever design parameters I've set up.
Cavalier: Stealth armor, 200 kg payload.
Raiden: Vibroclaw, 200 kg payload.
Longinus: SL/OS-SRM2 combo (i.e. 1+2 slots and 275 kg).
Fa Shih: Mag Clamps, Magclaws, 200 kg payload.
PzS: 10 points armor, 2 MWMs.
The PzS can't mount a heavy enough weapon to make a body mount worthwhile, so it doesn't get one. It has slots left, so it gets a SSWM.