try it out in the store, if you can, i got one at a discounted rate b/c i bought online, paid shipping blah blah and my little shrimp can clear it-no problem..
so it's been a waste of money and it's on craigslist right now-haha.
I have a 15 lb cat who stays upstairs most of the time, my yorkies give him a hard time when he comes down sometimes and he'll just go back up... but he enjoys wrestling with them some of the time. i think all animals have to learn boundaries and it takes a while for that to be in place... you have to give it time, if you can... you might try putting the cat food/box etc in a complete opposite end of the house or upstairs if you can, I gate my dogs downstairs with me -just because they get into trouble-but it also gives the cat some rest.....
good luck...