Research research....read some of Miles stickies in the stingray forum if you haven't already. If the ray is that small, of course you can keep it in a 150 gallon for around 2 years or so. Do realize that a motoro will need a bigger tank for their entire life, roughly around a 7x4x2. The bigger the happier your ray is (assuming your water is perfect!). As for feeding try some chopped night crawlers or blood worms. After that you can move them to whatever you want them to eat IE raw shrimp, pellets, fish fillet and such. Keeping a ray isn't as difficult as most people make it seems like, its just that you need dedication into making your water pristine for the rays.
I'm guessing your CK, and your silver is around 3"-5" ish. If that is the case you should be good as far as them living together but don't take my word for it.
As for an idea on water changes done on my 180 gallon, this is my schedule and keep in mind that my water parameters are perfect except nitrate at a low 5ppm-15ppm. I do around 10 gallons of water changes a day, 7 days a week alone on this tank. But I also feed them usually every other day OR a whole piece of raw shrimp per fish.
Your PH right now should be perfect for the ray. I will post some pictures of my motoros later tonight.
Any other questions you had in mind?