The ones that make pollen are males the ones that don't are females

I think that It's okay to top the mulberries off at the height you want but don't overprune. I have mulberries in my neighborhood and they're GIANT, while other types are dwarfs, so it depends on the variety. If you REALLY want them to stay small, you might have to sacrifice the three IF you know they'll get big. Hope your trees do well

Here's some Info I found

Pruning: No special pruning techniques are needed after the branches have been trained to a sturdy framework, except to remove dead or overcrowded wood. A mulberry tree can be kept to a tidy form by developing a set of main branches, and then pruning laterals to 6 leaves in July in order to develop spurs near the main branches. It is not advisable to prune the trees heavily since the plant is inclined to bleed at the cuts. Cuts of more than two inches in diameter generally do not heal and should be avoided at all cost. The bleeding will be less severe if the tree is pruned while it is dormant.

http://www.crfg.org/pubs/ff/mulberry.html

Further information: the best tasting mulberry variety is probably Persian Black. Many other mulberries don't really taste all that good, but in general the black-colored varieties taste better than the white or red. Pakistan mulberry isn't that bad and may be more productive for anyone in climate zone 10 where you do not get as much chill. The primary reason many people choose not to plant a black variety is because the trees can be very messy and the little black fruits stain.