Welcome to HRS Workman
as RawD said , you can make the traps, diffusers and baffles to help "tame" the room. Placement gets to be somewhat a challenge- and trial and error requires a good ear (a signal generator, or some room correction software)
as to the rock wool versus the rigid fiberglass, I personally find the rigid fiberglass easier to work with (cuts like buttah with an electric carving knife)
Will you be facing the short wall or the long wall?
I'm assuming the ceiling height is 8'?
One area that ill be problematic is the centerpoint area between the ceiling/floor/ side walls/front/back walls- here is where standing waves will be the biggest problem... and in a small room, probably right about where your ears will be
before you start- I'd recommend that you get a signal generator (there's freebie software versions to be found)- with your monitors set where they will be, slowly move a mic around the room (recording it) you should find areas where the signal is stronger or weaker- weaker is often a sign of standing waves canceling each other out from being out of phase. You'll need to "sweep" the frequencies so this could take some time, making a new track for each targeted freq- remember that harmonics come into play, so if you sweep 40 Hz, you probably don't need to sweep multiples of (80, 120, 160, etc) probably something along the idea of 40, 45, 50, 55, 60, 65 and 70 should cover it
OR get some room correction software, start with a pass with bare walls, and then see what/where with respect to adding acoustical treatment does the most good-
It is possible to kill a room by over-treating, so don't think you need to trap every corner and place diffusers and baffles everywhere- I have a lot of open wall/ceiling/floor space
here's some pics of mine, all the acoustical "treatments are built by me on the patio or dining room table (very understanding wife!)
