Well, you know, I think some people had the theory that the Gremlins/Mogwai were, but I don't think it was ever established. The tomatoes were not, but I figure Audrey II would have eventually found them to use in her war against Seymour and the rest of us humans.
One would think, kinda like Marvel movies owned by different studios. If you put Spider Man as a cameo in XMen, it might make purely XMEN fans want to check out Spidey and vice versa. Cross pollination and all that!