My '?!' are my own overused trademarks. You really shouldn't look too much into it.
But as a matter of fact, yes, I, for one, would have a problem if in a peace treaty with the Yeerks on Earth, they find their parasitic ways into the fauna. It would upset the natural balance of things, for one, and two, we all know this wouldn't last, for the tendancy is for Yeerks to want more feasible bodies.
I am trying to remember the kinds of feedback the horse-Controllers gave, but can't right now. I imagine that they weren't very fond of them. My argument is to do with the fact that they are after the dominant species of the planet, the fact that they are sentient or not is redundant.
The case in point would have monkeys with a sentient drive and the Yeerk way is not always in tune with the host's way, nor necessarily the environment. We all know not all Yeerks are the peace seeking agents we all would like them to be.
Willingly subjecting themselves into nothlits, may well have been the only solution to the conflict on Earth, really.