L.D.
I don't know anything about pressure-mounted gates, but we have wall-mounted gates that we installed. Like you, we did not want to damage the bannister, so we put a piece of wood between the bannister and the gate - the gate is then drilled into the wood and not the bannister. This has worked very well for us. Our daughter is very good about not going up the stairs, so we don't have any gates at the bottoms, just at the tops to prevent her from falling down the stairs. Besides, we figured if she does head to go up them, we can always grab her before she gets very far. But falling down the stairs happens in a split second. Saved us some money too because as you know, those gates are not cheap!
As for the cat situation, we have a fat cat too! We wondered how this whole gate situation would work for her, and really it's been no problem. We don't have to keep the gates closed at night when our daughter is sleeping, so kitty has full run of the house all night long. During the day, if she needs to get up or down the stairs, we can open the gate for her and let her through. And you'd be surprised - fat as she is (17 lbs) she can still squeeze her way through those bars if she really wants to!
Hope this helps!! : )