The four remix packages you downloaded are actually part of one large collection that counts as one add-on.  
The option to enable them is "UQM Remixes".

Ohhh, they're all ONE remix! That clarifies things! Thanks for that!

And as for choosing which HD to play, I would have to say that I am very biased towards the MegaMod. 
Not only because it updates the HD visuals to more match Vanilla UQM but there have been so many bug-fixes and enhancements. 

OK, I'll opt for the MegaMod as well. Thanks for your detailed answers and assistance! Exactly the kind of answer I was looking for!

First of all, great work! This is awesome!

So I've installed version 0.8, and I also downloaded the 4 Add-on packages. I selected them, and they seemed to have all copied themselves into the directory D:\Games\The Ur-Quan Masters\content\addons.

How do I select a remix? When I go into Setup, I see the option to enable/disable Positional Audio, 3DO remixes, UQM remixes, and Speech. But, I thought each of the 4 remix packs were different? I thought there were 4 remix packs: "Super Melee!", "Neutral Aliens, Don't Shoot!",  "Ur-Quan Hierarchy", and "The New Alliance of Free Stars". How did these 4 remix packs that I downloaded just turn into the two remix packs "3DO remixes" and "UQM remixes"?

Also, I see on sourceforge there is an "Ur-Quan Masters HD" (https://sourceforge.net/projects/urquanmastershd/) Is this the same thing as v0.8...but in HD? I also saw a Serosis Megamod (https://megamod.serosis.net/) that may be a fork of a later version of the Ur-Quan Masters? I'm a bit confused regarding how to determine what version of the Ur-Quan Masters a certain project has forked off from.