I always make it a rule to keep my species separate..even keeping cresteds (as adults) together is a risky business...males of the same species would fight and potentially kill eachother....
If you are thinking of keeping leachies with cresteds i would strongly advise against it....as the previous post mentioned, they are natural enemies...
Cresteds also have a tendancy to drop their tails if they are nervous. Which, in reality, isnt a big deal...but they dont grow back.
The fact of the matter is that unless you have a very large enclosure, phelsuma geckos tend not to seek out company...and both of the species may stress if you keep them together. stress=reduced immune system=illness....
but thats just how i feel..im sure others would disagree..
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1.0 ball python- Azazel
0.0.1 Yellow phase ATB- Devilin Hisseyes
0.4 leos- Boji, Sanjae, Fae, Azra
1.1.5 crested geckos- Jubeh and Inari, Avalon, Vajra, Pheonix, and 2 hatchlings
1.1.1 gargoyle geckos- Acacia,Osiris, and Enoki
0.0.1 flying gecko- Malarchy
1.0 leucistic texas ratsnake-Ojo
0.1 albino kenyan sand boa- Akasha
1.0 xanthic kenyan sand boa- Ashoka
1.1 whites tree frogs- Sifaka and Eikon
0.1 Transpecos ratsnake- Cleo
0.1.2 Senegal chameleons- Ambaji, The Chosen One, and Vertigo
1.0 Ruthven's X Thaeri king- Kemushi
0.2 pacific parrolets- Mishka and Kya
1.1 cats- Cairo and Oberon
1.0 crown tail betta- Lazarus
1.0 Husband (hes my favorite, but i dont let the 'kids' know that. 
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