Hi Annie
We have a moggie(Noodles) who has a very sensitive tummy and he can only eat White chicken/fish and tuna chunks with any brine removed(though he'd eat anything left to his own devices
) and now and then i mix a little pro biotic powder in his water(it's tasteless) and his poo's are great as is he,We tried and tested various sensitive tummy dry/wet foods which not only didn't fill him but didn't work for him although royal Canin Maine coone biccies now and then are for him fine,but i too would never use commercial cat milk or any milk as this i have found does nothing for them except keep their poo's even more loose and stinky
Hope this helps and may work for your cats