fbx is good, also another question, do your models work with mixamo animations?
So my rigs our unique, they follow the Unity Native setup for easy import, with some lesser finger bones, because even the most simple native rig in both Unity and Mixamo have too many spine & finger bones, my idea with these was to be as simple as possible so people would not have to do LOD's on animations and rigs down the line. But if someone wanted to run it through Mixamo's auto-rigger all they would have to do is remove the rig (And the skinned weights from my rig to save on weight calculations) then do the Mixamo Auto-Rig.
What I'm going to do though, is before the end of the day I will update this asset to include the Mixamo Rig as well. You're not the first person to ask for this, so I definitely want to update all my assets to have that option. But I have a lot of characters and I'm always busy with commission work. So if theirs any other models you've purchased from me, just send me an email or DM on Twitter and I can give those characters the Mixamo update too over the next few days :)