I've recorded many a brass band and this is what has worked for me. However, I'm just thinking off the top of my head here and since I don't really know your situation I may be totally wrong.
If you are recording in a large room without a stage, such as a band room, I'd place the group near the corner of the room with their backs to the corner. You don't want to get them all the way back into the corner, but you do want to have most of the room behind the mics. If you are recording in an auditorium of some sort, place the musicians near the stage lip and place the mic stand in front of the stage rather than on the stage. You will need a tall mic stand; a 4 m (13 ft) light stand will do. Start with the stand about 1 m behind the conductor, and move it up or down, back or forward to get the balance you want. Brass, of course, are highly directional, woodwinds and percussion are much more omnidirectional so you can use that to help get the balance you want. Two tubas with a group this small may be a problem as they may overpower, but this depends on the tuba players' abilities among other things.
Mid Side for the main mics is a fine idea. With a group this small you may not need the outriggers, but they can't hurt, I guess. Also with a small group if the room is good you could try Blumlein. Another technique I've tried with some success is NOS with a center omni fill mic. I'll mix in a little of the center omni to fill in the slight hole in the middle that often occurs with NOS. The center omni also adds low end weight that some cardioids need. Just mount the omni in the center of the stereo bar you use for the NOS setup.
Good luck and let us know how it turns out.