MRC01, nice to "meet" you--I'm another classical music recording engineer who hangs out here.
When you're recording at relatively close range in a generally well-balanced acoustic (i.e. not harsh or too reverberant), a fully detailed stereo representation down to the last centimeter is neither necessary nor desirable; you tend to get exaggerated width effects such as "the viola that ate Manhattan." Thus you can sidestep the defaults and use microphone patterns such as wide cardioid or omni. These can give a very pleasant overall "feel" of spaciousness to the recording, and musicians often loooove the results as far as their precious tone color is concerned (though my conscience tells me that this is partly a trick of perception).
I'm particularly fond of wide cardioids, since with appropriate placement, they let you get the nifty "feel" while still maintaining a stable stereo image. That is the best of both worlds as far as I'm concerned. On the other hand, if you're on a budget, the minimum cost of reasonably good omni microphones is far less than what you have to pay for a good pair of wide cardioids--unfortunately, since this means that a lot of people never get to use any really good wide cardioids, and never realize what they're missing.
If you decide to use omnidirectional microphones, their on-axis frequency response needs to be fairly close to flat in the treble. Thus you may need to do some shopping around, since the most popular omni microphones for medium-distance recording, and thus the ones people here would be most likely to recommend to you, mostly have a rise on axis of some 5 - 8 dB at high frequencies--a characteristic which is quite undesirable for close-up classical recording. There are reasons for all this, which we can go into if you like. Basically you should be looking for a "free field" rather than a "diffuse field" type of omni in your application.
--best regards