I find that the following human figures can be used together quite happily:
80s/90s CitadelBlack Tree Design (HYW, anyway - they're the only ones I have)
Curtey's miniaturesSome of the old Grenadier women sculpts from
Mega Minis (I figure they should be a bit smaller as dainty ladies

)
Mantic games skeletons
As for monsters etc, they'll all be odd shapes and sizes compared to 'people' so go with what you like the look of.
Hope that helps.