Stuff like the beagle, or superchargers, etc.

I have yet to run across a single Mech that has a mounting point for this stuff and I haven't seen it explained anywhere in the game.

I have found some mechs that have those slots, but up until now they're very rare (still early in the game, but I've salvaged about 60 mechs so far and found 2 with slots for superchargers).

figures..

they really botched the mechlab and customization part of this game tbh

Basically you need to know what individual variants could potentially mount certain equipment from tabletop to have an educated guess what equipment be mounted where. As for superchargers, seems their homebrew "P" variants of Mechs they added seem to be compatible with superchargers but don't quote me on it.

PGI did a terrible job explaining it to the player among other things of what can be mounted where.

I suspect there is a bug relating to some of this equipment that has to do with the Heroes of the Inner Sphere DLC. See the description https://www.gog.com/game/mechwarrior_5_heroes_of_the_inner_sphere That says it adds a bunch of equipment such as the Beagle Active Probe, Superchargers, and Rifles. I think they are supposed to be inaccessible in the base game yet OP has the first two and I have a few rifles. (I cannot find ammo for them though.) I have also seen superchargers for sale but since I didn't buy one I would not have discovered that they aren't a generic part like a heatsink.

Hence why I think there is a bug. Weapons and equipment are not gated properly but mechs and ammo might be?

probably what it is too

I've been stocking up on gaus rifle's hoping I find a mech that can use them.

I also see Hero mechs during the campaign as well.

Might be of use
https://docs.google.com/spreadsheets/d/15fQCjkxjECs3Jvhv-YA1ZP147SuEP2olkPzLJCLAFmQ/