The type of slime companion is selected in the companion settings menu within the pause menu, as mentioned above. Currently there is no difference between which you use besides how they appear, though that will change eventually. There will be a proper in game tutorial describing important information like this eventually as well, just not yet since there is still a lot to be done and a lot that is changing. A map is planned as well, they are just a bit complex to set up and there's a lot to do still. Same with rebindable controls, but I do plan to get to that one on the soon-ish side of things. Weight gain is also planned, but will need several thousands of new frames of animation, so it'll be a good while before I'm able to get to adding that.