As previously stated, the blue hippo isn't a good choice, and they do grow fast. IME, there are plenty of fish stores that would take them, but the amount of $ they would give you for them is dismal at best. The most I've gotten for selling a fish was about 50% selling value, but more recently I've only gotten $10 store credit for ~$80 worth of fish (different store). It's a crap shoot, but if you looked hard enough you could find someone that would take them. just to be clear, I'm not suggesting you get one. Just that they aren't impossible to get rid of. The blue hippos are probably the worst choice of tang for a smaller tank, but you could do a yellow tang.
I've had a one spot before and he did just fine in a tank smaller than yours. Full of personality, a fun fish to have.
I've always been torn between coral and non-reef safe fish. beware if you want coral down the road (if you don't already have any), you'll want to stay with reef safe fish. If not, you're open to what I think are funner choices for fish, like eels and small angels. I like them so much I've risked my coral a few times by adding "reef safe with caution" fish to my tank and have been lucky so far.
