Without seeing the room, here's what I would generally recommend:
Definitely out in the middle of the room, away from the walls. (Early reflections = evil in general.) By the way, this actually sounds like a pretty good room for this sort of recording, based on the dimensions, etc.
If you have the mics, channels, etc., given a choice between ORTF or spaced omnis, I'd go ahead and use both, along with maybe another spaced pair way out of the reverberation radius (at the ends of the long hall) to capture the ambience of the room, in case you want to use it. I never think it hurts to have options in mixdown/mastering, if you can.
With a group that small, I doubt you'll need spot mics for this, but, again, if you actually have the mics and channels, you could put them up anyway. If I had to guess, I'd say the harp would be the most likely to benefit from spot miking, since it will be providing a lot of the fullness of the recording in the lows, and also it is an instrument that wants clean, clear attack transients, etc.
This sounds like a fun gig. I envy you.
Regards, --Dan