Well, if David made an army, they would obviously all be humans because he doesn't have access to Andalites. I think if they went with the David distraction/demoralization trend, it is inevitable that the Yeerks would start looking for humans.
However, if you wanted to go that route, I would just have them all pick different battlemorphs than what they usually have, maybe pull something like they all morph the same animal and attack, giving the idea it's a different group of individuals.
When does this take place in the timeline?
edit: depending on when this is happening, they could have different ways to convince them an Andalite army is coming. For example, if it was after 40, they could probably acquire Ax, Mertil, and Gafinilan in different frolis maneuver combinations and go after them. Or, if you make them give up their "no morphing sentient beings without permission" rule, they could probably even manage to get a hold of some Andalite DNA in 38, or even offer Aloth/Gonrod a place on earth if they will fight for them instead of being suicided/re-imprisoned. Not that Gonrod would have been too terribly useful, but Aloth could probably lay some serious hurt on the Yeerks.