A list of all of Mario's Allies + Strategy for a Battle Team
To be sure Mario has all of his allies, here's a list of them.
Now, I first recommend a team for battles.
Mario can be your power attacker, Geno can be your magic attacker and Peach can be your healer. Mario's job can be to attack enemies and use some items if necessary. Geno can use his special moves like Geno Boost to raise your stats. He can also use magic attacks on enemies. Peach can mainly heal and use items.
Now, you are wondering about Bowser and Mallow. Well for Mallow, he can mainly do Magic Attacks. Bowser can just attack, having a lot of power.