Okay I hope this post will be able to clear things up;
1) If you're looking at a ray/shark tank, expect to be upgrading FAST. Your 100 gallon isn't enough for any of the species of sharks or rays. This leads me to point 2.
2) No rock is a BAD idea. You may be able to save space but the rock has a lot of phosphate eating bacteria that also converts fish waste into nitrogenous gases (harmless to fish). Let me put it this way, because a rock has so much surface area, in a 3 dimensional setting, it is far superior over the sand, which only has a 2 dimensional surface. Also don't buy live sand, its a waste of money in my view, because all it is is normal sand seeded with bacteria from LR, which presumably stays alive in the sand (but often dies off). So I would actually suggest CLEANING out your current sand, washing it in a bin and letting it dry over the course of a few days. Or just completely replace the sand, but from what I hear, money is an issue.
3) You want big monsters; sharks, rays. Don't expect that the tank will run well skimmerless. Big monsters also produce quite a bit of waste; which the protein skimmer often catches. I'd actually suggest selling your canister, then get a good hob skimmer (for price and quality I'd go with reefoctopus or a bit worse, but still good, aquac) and a few powerheads.