Depending on your sample size, you could do that. If it were me, then I would include more than one plant in each category. You will need a control (no poop, no fertilizer, nothing) group, a rabbit poop group, and a commercial fertilizer group. You should have maybe 4-5 plants in each group. So if you wanted to do small rabbit vs large rabbit, then you could have those be two separate groups replacing the rabbit poop group. Like the others said, I’m not sure if you will find significant results with that. You could certainly do that though. There is nothing wrong with having 2 hypotheses.
H1 : Plants grown with small rabbit poop will grow more than plants grown with large rabbit poop.
H2 : Plants grown with rabbit poop (small or large) will grow more than plants grown with commercial fertilizer.
I’m just wording those simply, but that is essentially what you would be testing. The word “more” would imply that there is a significant difference.