Top Contributors, knowledgable aquariasts please answer this question about stocking and compatibility?

Ok, I have recently set up another 125 gallon in my fish room. I have always wanted to keep a motoro ray, but not had the financial resources for their care until now. I have researched them quiet a bit, but am struggling to choose a tank mate. Most commonly suggested tankmates, ie arowanas, clown loaches, etc., are too large for this size of tank. I was thinking maybe a nimbochromis livingstonii, common names-kaligono, cloud cichlid, and the sleeper. I have also researched these fish quiet a bit, and think they may be good for my tank. They like sand substrate like motoro rays, like most of the tank to be open swimming area, only get to around 9 inches max, and are very peaceful, except to fish that can fit inside its mouth. I also like their hunting, stalking behavior to small fish. The only drawback i see is in water chemistry. Rays like softer, neutral water, while kaligono's like hard, alkaline water. I figured I could keep pH around 7.3 or so, and have medium soft to neutral