I'm planning on magnetizing all available weapons for my Crisis suits. I think I have the basics planned out, but corrections would be helpful. This should give an aesthetically pleasing result that is also functional.
1. Measure twice, cut once.
2. Drill a recess slightly larger than your magnet.
3. Superglue your magnet.
4. File off attachment points (tabs on Tau items)
5. Replace attachment points with magnets.
6. Repaint.
I use two sizes of magnets for my Tau. 1/8 inch wide by 1/16 deep, and 1/16 wide by 1/16 deep. 1/8 are for the bigger stuff (such as weapon load outs, shield gens, etc) while the 1/16 width ones are for the smaller stuff (targetting arrays, drone controllers, positional relays, etc).
What I found works is to simply take a 1/8" drill bit and drill straight into the forearm of the crisis suit, using the already available indent as a bit guide, same for the shoulders. The biggest thing to remember is to keep your polarities straightened out--it sucks having that one crisis suit who can't hang on to anything because his polarities are backwards. I've went so far as to using 2 of the 1/16" magnets in the bottom forearm of the stealth suits and two in the top of the stealth burst cannon and stealth fusion blaster (each magnet is mounted side by side, not on top of one another). I've also mounted my broad side railguns, too (sucks having those things break off, so magnets instead).
I've found mounting the magnet just deeper than flush with the model on the shoulders and/or forearm, then going over it with model putty, sanding it down, gives it a real clean appearance.
As for the original topic, isolate what you need to swap. As mentioned, tac squads aren't going to change much (maybe magnetizing the Sgt and a few different weapon load outs for him, and the heavy trooper in the squad as well). Magnetizing assault marines to accomodate jump packs or standard back packs isn't a bad idea, either. Also, as mentioned, your dev squads.