[@Drakeonis] So many good questions. :)
1-Yes, BanchoLeomon is off-limits. [Sub]That is, until he dies like a Leomon in almost every series.[/sub]
2- In the case of Mervamon, she could serve as an Ultra form, while Minervamon would be its Mega form. MadLeomon would be an acceptable Dark Digivolution, and could count as an Ultimate.
3- I suppose that'd be fine. The Leomon in Tamers pulled it off.
4- Seeing as how Gatomon is one of the more popular Digimon, thanks to Adventure, I'd recommend using either one or the other.

[@DarkwolfX37] Well you could use Fusion Beelzemon. I would consider them separate entities.