Another entertaining story route is if the characters join the respective groups, but then are forced back to working together for some reason
<Snipped quote by Sylvan>
This is gonna both great and frigging terrifying! I wonder how long I can survive with two irregulars XD.
If by custom you mean connecting together logical steps to create megas that have none (e.g Gundramon) Then yes, Unless you mean like full custom digimon then no. (By that I mean, A totally OC digimon, Not like er for instance linking Beastmon to Lilithmon etc.)