Are all these devices connected to the back of your Mac Mini directly? It's not a great place to have a wireless USB adapter, so you might consider finding somewhere less obstructed for it, for example if you have a wired keyboard with USB ports you might try one of those, or you could get a USB extension cable or hub.
I had a similarly restricted setup and ended up getting myself a long (3m) USB cable and using it to allow me to mount a hub on a wall in a nice convenient location. This not only made it a lot easier to quickly plug in devices like USB sticks, phone etc. but it also solved some stuttering with my wireless mouse. Of course it all depends on your setup, and how neat you want everything etc., also, you don't want to go longer than 3m (mine is actually 3m + a 1m cable for the hub) as USB's wired range is around 5m, so you might get worse interference from that instead; nothing worse than trading one problem for another
But yeah, if the receiver is on the back of the Mac Mini then it's most likely being obstructed by the other devices; while it can certainly still work like this, you really want it aimed towards your wireless mouse/keyboard if you can, as even when a wireless receiver seems to be working you can't guarantee you won't get a sudden bout of interference.
To answer the original question though, the Mac Mini should definitely have enough power for a USB device in every port, and should also handle even a couple of hubs just fine depending upon how heavily loaded they are exactly.
