my cats (and dogs) love the kitty tv (a 75 gallon fish tank in the livingroom)
the cats also LOVE the heat from the lights, they frequently sit onto of the tank snuggled next to the light fixtures...this is why a secure lid is a must...
i wouldnt go smaller than a 10 gallon, not only because of stability but because anythign smaller than a 10 gallon is useless for anything other than a betta fish lol
i would go for a proper tight fitting hood (not the glass covers but a well fitted hood and light) if you cant get a tight hood buy one of the wire tank covers form the reptile isle and sit your lightstrip ontop of that. and a SOLID stable stand (or on a serious desk ect...not on of those flimsy looking mettal stands...
on the "mean to the fish" aspect...ive never had any issues and ive always kept fish and always had cats. when introduced to a new tank fish will bolt and hide, but once theyve settled...
I make sure my tanks are filled with lots of hiding places, but ill be honest when i say the fish dont seem to bother...ive had fish that actually CHASE the cats and dogs when they paw or sniff at the tank lol.
my dominant angelfish will chase, my other fish tend to ignore the animals...they know there safe in thier big glass box