Im also new to stingrays, but from what I have researched so far I think you can keep silver dollers with them.. And as for a stater ray, they say motoro, but most people dont have the tank floor space for one so a hystrix would be good but are hard to find..
That tank will be fine for awhile for rays, as long as they aren't huge rays already.. they'll love the width and length it has.. They loved the old ray tank I had that was 5x6x17".. It wasn't perfect but they loved swimming around in there.. Eventually I'm gonna buy a 300 gal Rubbermaid stock tank for a garage ray pond for more room and rays..
Hey if you haven't tracked down any rays yet check out Michael at Aquarist World, he has lots of Motoro pups, and John in Moses Lake has some real nice Mantilla pups and a Motoro he said he'd throw in for $65 I think if you buy a mantilla.. That's where I got my Henlei from.