You do not have to worry about sex on radish flowers. What you must know is that you have to pollinate from different plants meaning, the flowers on the same plant will not pollinate each other. So it is easiest to just have a patch of radish plants that you allow to go to flower and the insects will take care of the rest. Radishes need about 1/2 mile for purity of the variety.
Being you want to grow multiple varieties and hand pollinate, that is difficult in my opinion because you need bag the blossoms of multiple plants and then hand pollinate while worrying about insects showing up while doing this.
Since radishes are not a long season crop, you are better off staggering plantings so one variety at a time is blooming. Then you need to do nothing but collect the seed when ready.
Remy

I agree with Remy. Do a 'time isolation' of the varieties. Plant them @ 6-8 weeks apart, or longer if possible. Or if it isn't crucial that the varieties be true-to-type, grow them all together, let 'em cross up, and start selecting for what you want in the next grow out. Or, let 'em cross up, and begin your own multi-colored radish grex!

Winston