It doesn't matter how many tanks you have, you're always going to need more! But this isn't a bad thing, just a case of getting the balance, especially for guppies.
A good friend seperates his males from the girls then selectively adds the male(s) he wants to the girls tank or a seperate tank for a few days, then wait and see what appears from the batch. He's good at sexing fry when still very young , me I'm still getting my eye at this stage for sexing at the 2 week mark, it can be done, just takes time and patience.
I go for the other option, keeping the strains I want to keep in their seperate tanks, I then move any fry that appear or seperate the pregnant girls into a seperate tank for their drop and then raising the fry. Just takes time and patience but I'm finally getting some good results on both our blue grass and the metallic pinks. First new pink male coming up an absolute gorgeous colour which we've been after for some time, plus another drop of fry which have just gone into the same tank as the galaxy rasboras. Only thing I've noticed is some strains are very very slow growing, the albinos especially.