What are your water parameters? This could be a clue as to why the guppies are dying. If it is, you'll need to get it straightened out, or the cats will die, as well. I'd check out the water quality before I added anything else here. As for the cats' potential size, I'm not sure (surely someone will chime in here), but you'll need a bigger tank. Ten gallon tanks are highly susceptible to temperature fluctuation, water quality problems, and overcrowding. Upgrading tank size ends most of the issues for you.