At the moment there are no plans to allow more than 2 characters in the party, both because the actor art will start to overlay the enemies on the screen too much and because it will require a lot more thought into balancing encounters. This stance may change in the future though!
I am not sure if I will allow Heroines to join your party but there are plans to let some of your kids join as minions instead.
The Snail just returns to your home when you dismiss him. You can find him loitering around outside your house in Emerald Falls. He is kind of selfish, so you will need to give him another Cabbage in order for him to join your party again.
Glad you're enjoying the game! Next update will have a new romanceable Heroine with her own quest line (at least if everything goes according to plan).