The autocannon shots (and likely also cannon) hit the legs of the spidertron that fired the shot. Which is what vanilla cannons do when fired from tanks to spidertron legs. Looks like it might be because hit_collision_mask on the legs have the unset vanilla default value, though changing that might cause other issues, unsure, untested. A new custom collision layer that all cannon type shots being moved to would fix it though for the legs.

Please find mod option "Force condition Autocannon shells" under Schall Tank Platoon Startup tab. Set it to "Not same" or "Not friend" will avoid the self collisions. (Similar option for Cannon shells.)

Adjusting collision masks is quite bug prone (as you mentioned), because many mods love to modify them. Different mods may use unnatural hacks (say, some special collision layers). so mod conflicts are inevitable when they try to modify the same entity.
In contrast, applying "force condition" to projectiles is much simpler.