You may want to note that Onix is a Level 2 Acquisition, meaning you have to make it your uplevel, or drop (Or replace) it. If you do make it one of your uplevels, however, you shall have to take uplevel status away from either Pidgeotto or Grovyle.
Also, we do not really have different stats per Pokemon in ASB. Generally, larger Pokemon are bulkier, while smaller ones are more nimble. The Species and Type characteristics would shed light on them.