Like it or not these type of extremely cramped set ups are quite common. There's three main issues that come to mind with these tanks.
1. Toxin build up....BUT....if you carry out the crazy amount of water changes needed to keep the toxins constantly minimal then the water could be as fresh as it is in our tanks! So, mega overstocked or not, the water's fine. Box #1 ticked.
2. Compatibility....BUT.....if you get really lucky and all your fish get on then that is box #2 ticked.
So, so far, with a crazy water change schedule and a fantastic bit of luck, your mega stocked tank is ok?
Then we come on to the third issue, which imo is more serious than the other two, mainly because, unlike the other two, there's nothing you can do about this one, other than upgrading, and that is.....
3. Swimming space....this one, for me, is the saddest one. You can kid yourself all you want regarding your crazy water change schedule and enormous stroke of luck regarding compatibility, but it's the amount of room the fish have to swim in that make these type of tanks so shocking to me.