Your chiclids may pose a problem depending on the type. With wildcaught rays there is alot of things that can go wrong starting with the way they are caught, handleled, exported and acclimated which will lead to alot of ray deaths before they even get to your house and even when you think you have a healthy ray in your tank it may have had some damage from any of the above that may cause problems later, not to mention any disease or parasites from the wild.
Sometimes buying a bigger ray can cause more problems at the start due to a tank not being able to handle the bioload of a ray that size so to me a ray around the 3-6 month age will usually be the best to buy for a begginer.
If your not planning on a tank upgrade you may be able to buy a smaller species of ray. The 2 hardy species in the smaller rays is the scobina and the hystrix but you won't be able to find any of these around here probably. The other is the rectics but I don't think you will be able to find a captive bred ones around here but should be able to find some wildcaught ones. I don't reccomend buying any of these for most begginers as they are usually the most abused ray during shipping.
As far as what to look for in a ray to me it's all a matter of personal preference for looks as health wise make sure you see it eat, check to see how thick the base of the tail is, check the disc edge for sings of rotting or it being thin, watch it's breathing pattern, see if you can see any of it's feces in the tank to see how it looks and check it externally for any sings of parasites or know the breeder and trust him and you won't have to worry.
