Get the gardevoir a mate of her choosing. That usually works.
In all seriousness, this sometimes happens with male pokémon, and there are several ways you can resolve this. You can get the gardevoir a mate as noted above, you can neuter your lucario and greninja (although this can be trickier for the latter), you can mediate, or you can simply train your pokémon to refocus their anger towards more a productive means (such as battling) while also teaching them to resolve their differences peacefully. The most extreme solution is to get a mate for everyone, as not only does this double the size of your team and thus double its needs, but also, this could encourage them to, ah, set aside their differences, so to speak. Which could be worse. So only use this method if the above several don’t seem to resolve things, anonymous.
You may be wondering why I’m not addressing the egg group issue. The truth is, simply, egg groups only refer to the logistics of both the act of mating and the production of a viable egg. Pokémon themselves, at the risk of putting things crudely, don’t care about group boundaries and may attempt to copulate with anything that they consider to be an attractive pokémon.
You’re welcome.