Is adding GAR and/or MP to some HQs (if you already have HQs in the area) worth it for partisan surpression or is it better to add INF in this role as well?
Also, it was a while since I played, what is the most effective way to build units regarding the conscription/mobilisation laws and are the air, land an naval units affected the same by it?
The first part is subjective, depending on which country you play. GAR has decent Suppression, two points, and is fairly tough on defense, although they're too slow for many fluid situations with a Corps HQ. They can be fine in an Army or AG HQ where you rarely move them, and they can be Strategically Redeployed to wherever they're needed. Adding an MP brigade for additional suppression is optional, and can be helpful in a few key supply bottleneck areas (like just below Lithuania, if you're playing GER). MPs have the same problem as the HQ itself: they can't fight without a line brigade. MIL has zero suppression, so it's only role is to protect the HQ, but the HQ will potentially move faster with some research, and the MIL will hold it back. Worse, a HQ with MIL will remain in combat and probably lose badly to just about anything, rather than auto-fleeing. INF works well enough, giving you one point of Suppression and enough combat power to hold off a Partisan attack.
Again, the second question depends on country. If you've got a long time before your first confrontation, switching to 1 year conscription or volunteer army can allow you to build up a huge infantry army before the war starts, and you can mobilize it up to its full strength at HALF the difference in cost between un-mobilized and mobilized for your FIRST mobilization. That won't work for Italy, for example, which is already at war against Ethiopia at the start, or if you intend to do an early conquest and then demobilize (with something like HALF of the demobilized manpower being returned to civilian life and no longer available for conscription). Air and naval units are only buildable at full strength, so there's no savings for building them as Reserves at reduced recruitment laws. I'm not positive about armor, but I don't think they're available as Reserves either.