Hi there!
The 3PC&T course has just added a lecture about ‘fixing’ Mixamo animations. With respect, some of the info presented is incorrect. I don’t know if there’s any actual difference between the two sets of rig names, I haven’t really compared. But the “Mixamorig:[name]” format has literally been there since the beginning. We worked with them in their early days in 2013 and AFAIK that’s how their auto-rigger has always named things.
I’m not sure what changed between the two characters you downloaded but the rig name doesn’t change by re-downloading the character, it’s just whichever the character is using (you can upload already-rigged characters just for the animation matching). The character I chose, Akai, uses the “Hips” etc. format. The animations you download will get set up according to the character you have selected at the time you download them. From what I’ve seen, using the same character for the downloads, and then also setting their avatar definition to “Copy From Other Avatar” using the player model’s avatar, will generally get things working well. I’m gonna try and stick to this and see how the rest of the course goes.
That said, i HAVE had a lot of issues over time with some animations looking like **** on certain characters when the models are too different, like using a skinny human’s avatar to play animations made on that Brute or the Mutant for example. I think some of the running animations I tried for this course actually gave me lots of weird twisting in the heels that I just couldn’t fix, so I just grabbed several to try. Eventually one of them worked and looked okay.