With guppys in a 70 litre, I'd go with `1 male and two females only. After a couple of months you will have 25+ fish, and it's time to re-home. I'd trade in the lot at your LFS, in exchange for 3 new ones, 1 male 2 female again. I say trade in the lot, as allowing siblings or parent-offspring breeding weakens the gean pool, and increases the risk of deformed offspring.
I assume that this 70 litre has sufficient filtration for adverage stocking, therefore 25 is the most I'd allow in there, as this brings you up over 2 inches per gallon. Guppies are low waste proucers, so it is doable, but you would have to be prepaired to move fish on.
At the LFS in which I work, one customer set up a 4 foot tank. 1 year later he had 3 1 foot tanks, all full of only guppies, stocked to close to 3 inches per gallon. Needless to say, he needed to get rid of a few, so for about 6 months, we were getting local bread guppies only. They were nice, with a bit of endlers live barer in them. Just goes to show how quickly they multiply, as he claims to have only purchsed 6 as "starter fish" from one of our competitors. He has since left the hobby, so we don't have any local breads any more. The imported stock we now get is very shoddy in comparison, even though inbreeding was evident in the fish he was supplying.
HTH
Rabbut