The CSS65 will fit in a 10, however, it will be pretty cramped and you won't be able to get more than maybe 2 baffles between it and your return pump assuming your return pump is submerged. As to the 15 gallon tank. There are two sized 15 gallon tanks which is where the confusion is coming from. A 15 tall has the same footprint as a 10 only its 50% taller. A 15 regular's footprint is 12" deep and 24" long so it wouldn't work. The 15 tall's advantage is only that it will hold more volume of water to prevent flooding when the power goes out. It will also be a little more difficult to access stuff in your sump depending on the height of your stand.
I would also recommend hanging the skimmer off your tank. It should fit, but I would strongly recommend if you go this route to build in an overflow tube into your collection cup and have it empty into a container over your sump because it will eventually overflow.
I've got a CSS65 hanging on my 10 sump with 4 baffles with a return pump (mag 3) and it works pretty well, even though when the power goes out the sump water level rises nearly to the top.
Good luck, 29s are great tanks