In the midget niche I wouldnt split it up too much - there is very little traffic in that niche but it is golden if you can do it in a nice way - same as with the bbw niche - people that like bbws "love them" and wont buy if you push women eating food in bed or calling them gross fat pigs - take a look at the way MML pushes BBW and you will see the right way - anyway the same goes for midget - dont degrade them in any way and you should be able to make sales - however keep in mind that it wont be a bunch of sales every day - a few galleries in a week would flood the market
as far as MILFs age - its been discussed here before - dont get it confused with the mature category - MILFs are mothers - plain and simple - any age from the legal age in your state for being a mom - up to 80+
grannies and older women is a completely different niche - and is usually broken down into 30+, 40+, 50+ and grannies - although to me 30+ is spring chickens LOL and 40+ are just starting to catch up with me - at 50+ I might call them experienced at my level