Seems like if the bone topology is similar you should be able to map different bone names to the same common name between all the skeletons you buy. Sometimes there may be a different number of spine bones, neck bones, arm roll bones, so that could possibly cause some issues.
Maybe if you have Maya or something, you can open the files somehow and rename the bones manually for now. It could be possible to add this feature to Unreal. Like a bone name mapping table that renames the bones upon import.
Different skeletons also have all the joints located in different areas so it may be best to try to keep a different skeleton for each model but just rename them so the animations all work and you can use one common anim blueprint in some way.
I think animation retargeting would then allow you to reuse animations from the different skeletons, although I never tried using that yet so I’m not 100% sure how effective it is.
Personally I’m not going to have this issue since I’m not buying any characters but that’s not always feasible for everyone. I also don’t like purchasing characters so much since you can’t keep a coherent design and personality. That character is likely to be reused in other games as well and people are going to see the same guy in multiple games. Maybe try to stick to only using Mixamo characters or something so you don’t have a jumble of character styles and types and skeleton structures.