Question how do I get the slime girls to help and fight along with me? Or even make them adult?
So you can feed them daily with Slimeballs. After they eat enough, they will reach their "adult" stage. After that, you need Supercharged Slime in order to make them grow anymore.
Go to Joe's House (the slime breeder). Either help him with his task 5 times or eat him and fiddle with the control panel. You'll see a cave out back with a large slime. You need to feed her Jello Slimes (get the Capture Net, go to the Forest and Capture Them). When you feed her enough, she'll give you some Supercharged Slime. Feed that to the Slime Girls. Once you give them enough, they'll size up again and you can then take them into battle with you (by talking to them and saying "Come With Me")
Hope this helps.