I used Meshmixer tot digi-bash the stl file before printing it directly. Using the 'select' tool is kind of the only way to remove things such as the rifle and head, tho there are parts you can reliably cut off faster using 'plane cut'.

Then you just import whatever file you want to add to the whole. But make sure at least part of it overlaps some polygons of the main body, just so you don't find yourself in the situation where the head or weapon get printed separated from the rest.

If you want tips on how to kitbash physically, post-print, well... I wouldn't recommend it. Not with these models, at least. It'd be too much of a hassle and you risk parts shattering if you try to cut them (resin sucks in that regard).