I dont know how the geckos would take that sort of stress. The geckos live depending on species in bushes and trees and hardly ever if not never come in contact with a body of water deep enough were they would have to swim. I never seen one but they most likely do not make good swimmers being arboreal and all. I dont know what species you were planning on using but the smaller species wouldnt be able to take a cichlid biting on them when they do fall in the water and get mistaken as a piece of food. If you want something like this a smaller body of water like a small creek like area just shallow enough for them to stand in might work. I wouldnt mix them with mantellas either just cause of their skin secretions and some of the bigger uros might be able to take down a mantella.
-----
Center for Reptile and Amphibian Conservation

Reptiles

1.0 Uroplatus phantasticus
2.0 Uromastyx malienisis
0.2 Eublepharis macularis
1.0 Chrysemys picta dorsalis
0.1 Physignathus cocincinus
0.0.1 Hemidactylus turcicus
0.1 Lepidactlyus lugrubis