Great game. Like others said here you should definitely gain an extra baby slot after the first floor. Feels like you've only got one slot where you can really variate since cube of meat and lil chad are kinda mandatory and you need one slot for a shooter.
I agree on having a 5th slot, but want to mention that Bob's Brain is a good alternative to Cube of Meat. It's cheaper and kills enemies, so you can save more hearts and defend with them for a while until you get all of your Lil' Chads. At that point, you just start placing rows of your other units and use Bob as a cheaper, explosive meat shield whenever you need one