Rules, I can have Doctor Who figures but only the Doctors and only the 5" scale because I already have Star Trek figures that size. If I wanted a whole army I would have had to forgo something else completely like the Monsters.
If all else fails tell yourself you will not become that woman on the hoarding TV show with 1000 dolls.
I also got in to the idea of multiple heads sharing a few bodies. It works for mid to lower level characters you don't really use that much.