If you want shellies I think you'll want a sand substrate. I use pool filter sand. Works great, easy to clean before using, and is relatively cheap. Only down side is that it only comes in one color.
As to who you should put in your tank, I'll defer to those with more experience, but I will tell you that I have 55 with shellies (melegrise), calvus and Cyps that works well for me.
Good luck.