Generally it is always a bad idea to mix species. They may encounter each other in the wild, but in a captive setting it just doesn't make sense. You could do breeding harems with crested geckos if you had a larger enclosure 1 male to as many females that can fit comfortably in the enclosure. However you need to watch for gravid females and aggression between cage mates, as the males will relentlessly try to breed and sometimes the ladies won't tolerate it Also, The females can have their calcium stores completely sapped from laying, which can cause many health problems. I never house my geckos together unless they are breeding, then I remove them after a month or two. That way each gecko gets enough food and I can monitor their health with ease.