TamirWasn't the FreakPugs system just like PugChamp? It's basically what I'm trying to avoid in order to make the competitive scene more welcoming to new players.
Yeah, frkshw and I literally copy-pasted the source code for PugChamp from GitHub and threw it up on a free Heroku instance lol. No programming knowledge required; shout-out to HANK for providing solid TF2 servers. Our goal was to make a place for UGC and low open players to pug since there was almost no way many of them would get picked on PugChamp. People seemed to really enjoy it overall, but it turns out that tsc did not like what we did (which was understandable), so he added an explicit "copyright notice" in July 2017 to restrict people from making clones: https://github.com/fwdcp/pugchamp-legacy/commit/2298c51d70c35abf71e5fe8a31979e0d834f378e
Brief history lesson aside, making a TF2 pug/mix site is very difficult when you realize that the relatively small size of the competitive scene means that you have to seriously consider the tradeoff between getting pugs running "quickly" and keeping pugs fun and evenly matched. People don't want to wait forever to play a pug, but they also don't want to finally get picked for a pug only to get rolled 5-0 in 15 minutes.
In my opinion, the "ideal" mix site would keep 4 skill ratings for each player (corresponding to skill on scout, soldier, demo, and medic) and disallow switching classes with teammates through plugins. Creating a good team balancing algorithm that takes these ratings into account is challenging if you are set on restricting the pool to 12 players for each pug, and it becomes even more challenging if you decide to implement a system where medics can choose a particular player to play with. You can simplify things by keeping only 1 rating per player, but the underlying issue remains.
In general, any mix system will produce lower quality pugs than a captain draft system due to the fact that it is very hard to establish accurate skill ratings for all players in the pool. Therefore, it's better to have trusted captains make the teams, but if you are targeting your site towards people in "low divs," they probably won't know people very well and will just pick their friends even if this would make the pug noticeably imbalanced.
I'm not trying to discourage you from working on this project, but I wanted to make sure you understand what you're getting into. Setting up a mix site worth using will take a lot of effort, and much of it may go unappreciated unless you knock it out of the ballpark from the get-go. Best of luck.