you forget that even the weakest character from Dragonball Z (oops i meant the weakest Z fighter) would be capable of taking out the entire Andalite fleet (of Dome ships) with BOTH hands tied behind his back. Even Gohan, when he was very young, was AT THE VERY LEAST like a living Dome ship in terms of power.
Debateable in the earliest episodes because at that point Gohan had no idea how to use his power and even when using it couldn't do what Dome Ships could do, which is take chunks off of planets. Vegeta on the other hand, could destroy an entire planet so I'll give you that one, but really a blast from a Dome Ship would have incinerated any Z fighter in the early series, spare possibly Vegeta and a revived Goku.
But regardless of where in the series the Yeerks might have shown up all it takes is one yeerk in the head of the right person and the Z fighters are fairly screwed. Goku wouldn't exactly be on board for say, killing Gohan if he got infested and if they don't know the yeerks are a threat all it would take is say falling into the yeerk pool. Infest one and before the three days are up get another of relatively similar power through trickery and have them restrain the first when their yeerk is being fed, with those two it's possible to then get a stronger Z fighter on up the list, they'd be tricky to hold on to of course until they had most of them but given that DBZ was somewhat lacking in powerful minds (don't get me wrong, I love DBZ, but Goku was
so blonde sometimes) they might never notice any change in their infested friends.
If all else fails do what the Yeerks usually do when they can't get a certain host: infest lesser hosts to get to the one they really want. They did this with Aftran's host, Taylor was a poster girl, and there was the unknown girl (I like to imagine it being Taylor, just because) who lured Tom in. If the Yeerks got, say, Bulma at any point after she'd gotten together with Vegeta they could easily infest her child, a properly clever yeerk could get at Vegeta too. Heck Bulma was even close enough to Goku that she could convince the poor guy to put a Yeerk in his head willingly if he didn't know what it was, make up some story and after all they'd been through why shouldn't he buy it? If the Z fighters aren't met by the Andalites first as the Animorphs were, they, like the Animorphs themselves, would have been in trouble just because the Yeerks aren't as obvious a threat as the ones they're used to.
And if the Yeerks get Goku and Vegeta that's pretty much it for the Z universe, and you could get both just by infesting Bulma (After she got close to Vegeta that is . . .) and honestly I don't really see Bulma being very difficult to infest or hold on to so they wouldn't be in any rush until after they infested either Goku or Vegeta. Over three days since infesting Goku or Vegeta the Yeerks could use either one or both to infest everyone else forcefully and thereby have a way of restraining the two while their yeerks fed or indeed swap yeerks instead of letting the controller feed so that there is minimal down time, as is shown in
Visser a Yeerk can be mostly out of the host and still not relinquish control and Z fighters
do have two ears. One Yeerk goes out but doesnt give up control until it feels the other Yeerk assume it.
Of course maybe they'd sense the Yeerk in an infested friend, however even then if they don't know what the Yeerks are they might not recognize any danger so while I won't say that infesting the Z warriors would be easy by any means, it isn't quite impossible. As with most things you just need the right strategy.
