With preds, as mentioned, you really need to be careful what you mix, and how you mix (order of acquisition/size differential). Some fish beat each other up, and some will simply plain eat each other, yet others get along just fine.
Unfortunately, for even the smallest shark or ray, you need an open-footprint tank of at least 300 gals, generally speaking with very little in the way of decor, as they tend to get stuck in between the decor and the tank walls. A soft sand bottom is also required so they don't irritate their soft bellies.
Once again, research is your friend...it will save you a lot of grief and give you the best chances of an enjoyable experience. Make a list of fish you like and research them or toss it out in the forums for opinions/review.