Abillity to upload custom cards and play custom formats?

I've just come across twinleaf, and I'm excited for the promise it brings.

I wonder if twinleaf could also be a solution for a project I want to test out. I'm making my own custom Pokémon tcg format of 2000+ brand new cards that are balanced, and only interact with, eachother.

It would be great if twinleaf allowed for uploading of custom cards. Alongside with tweaking certain rules, like first player starting rules, that are already required for playing older formats, that would be perfect to playtest my format with more people quickly.