You're assuming 3 things:
1) that all other species of alien can reproduce with Saiyans (unlikely)
2) that all other species have sexed reproduction, which, again, unlikely to be all of them, some of them could be like mushrooms and have like 36000 biological sexes and reproduce by spores or parthenogenesis or roots...
3) that even if there is a compatible species with males and females, that the females of that species would be weaker than the Saiyans, which, again, not at all a given. There are plenty of species on earth alone where the female of the species is bigger, stronger, more agile or more aggressive than the male, it's actually about a 50/50 in dimorphous earth species on which common sex is bigger.
So, if you want to write your half-saiyan fanfic where there are a bunch of half-saiyan kids around, go ham, but also you don't need anyone else's opinion to write the fanfic, you can just write it. If you're inspired by howin history colonisers tend to pillage and assault the people they colonise, and that assault sometimes results in a baby, you can write that dark story too.
Just, word to the wise, if you're talking about human women, don't call them "females", I get the sense you do that a lot. It's kind of dehumanising, and people, especially women themselves, don't tend to like it.
