You're absolutely right... in general, working with a pre-existing group (whether it's an orchestra, band, or a chamber group) is MUCH easier than recruiting individual performers and slapping something together. And to work with such a group, all you need is one contact (ideally the conductor), so U238's point seems like kind of a non-sequitur. Generally, community and school groups tend to be relatively open to performing new pieces (as long as they are relatively easy from a technical standpint). With professional orchestras, it's obviously a different story, but often they will have competitions where you can submit scores for readings or performances (the upside to these is that, if your music is good enough, you don't really have to "network" beforehand to get a good performance... but having the performance and recording under your belt gives you more credibility and, I think, makes it easier to get other performers interested in your work.)
I feel like this leads to one of the central paradoxes of being a composer: to get good performers to take you seriously, you often need good recordings of your music, but to get good recordings, you need good performers to take you seriously. This is why competitions that offer a performance are an extremely valuable "foot in the door" to young composers... and why I think the DanDJTitchener's idea here is a very good one.
On the relatively-unrelated note of young composers writing for large ensembles, I'd say that it isn't as clear-cut as U238 is making it out to be. We write what we hear, so if you grew up playing in a wind band, you're probably going to start off writing better band pieces than string quartets. Yes, there are elements of orchestration and balance that take years to master, but the same can be true for chamber music... in the latter case, you have to do more with less, which can take a lot of control-- and can be very daunting if you tend to think in broad gestures and varied colours, which you probably do if you play in bands and orchestras and are used to hearing that kind of music. (Steven Bryant, a relatively successful band composer, mentioned that he is terrified of writing chamber music and tries to avoid it at all costs.) There's no one right approach, and I think that kind of prescriptivism has led to the artistic downfall of many a young composer.