Board Thread:General Discussion/@comment-7052631-20130415000542/@comment-67.233.243.140-20130415025834

Actually, I do not think that he ever stated that all the species in the Milky Way are in the Omnitrix, not even all the intelligent ones.

Anyway, a requisite level of intelligence is the main criterion. As we've seen with the Nemetrix, it is dangerous for sapient beings to inhabit the bodies of non-sapient animals. Exactly how much intelligence is unspecified, though in Primus, we did see Azmuth, reportedly the most intelligent being in at least three galaxies, adopt the form of an Appoplexian, who are generally considered quite stupid, without problems.