Well to start with, while you can get characters from sketchfab, it's not really a character creator, it's a hosting site for showing if your models, those characters could be made in any 3d software and in any version of any if the supported export types, so there's not really any compatability to aim for, it's like saying you'll make it compatable with the game creator store.
I would suggest aiming for matching compatability with the export types and quirks of 3ds max, makehuman, and blender as they would be the main three used . Mixamo is another common one, but as synchro said, how long it will be around is anyone's guess because it was a product bought out by Adobe, and we all know Adobe loves to buy companies out specifically to shut down their products and kill competition, and now that fuse is shut down mixamo is probably on its way too.
Personally I create everything with blender, using a standard bone rig with IK control bones, painting weights to the vertices, and animating multiple animations on a single timeline.
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