Depending on the species and thier requirements, some animals can be housed together however, your set-up is not made to do this and does not take a number of factors into account such as temperature and niche requirements. You have made the tank and now are trying to shoehorn animals in the percieved niches in the tank instead of choosing the animals ahead of time and creating the niches for those animals. For example you have basically a humid semitropical setup and are keeping a desert species in it (the fan footed gecko....)this is not responsible herpetoculture.
Using the fact that they have been okay for 6 months is pretty meaniningless when the life span of those animals can be a decade or more.
Before you proceed any further I suggest you peruse the following thread as there are multiple issues you have not taken into consideration.
http://www.dendroboard.com/phpBB2/viewtopic.php?t=4532&highlight=multispecies
Not to be insulting but you referenced "when done intelligently" and your setup does not do so otherwise you would not be making statements like "maybe the salamander will survive". You have created a set-up and are simply adding animals to it without regard for the animals' wellbeing....
I highly doubt that the temperature at the bottom of the enclosure will even be close to the proper temperature for the caudate and you will be simply dooming an animal to suffer a long and agonizing death for no real reason but that you want one to live in the water....
Ed