Kevin Brown's shop seems to know anything that was ever made Omega wise for me locally, then there is Harry of Harry's Classic Enlargers, but I am not sure if harry supports the newest Omega stuff. The smaller format mixing chambers in my experience do make for faster printing on smalle than 4x5 negs, however, this is more aimed at production printing in my book.
I have always used the 4x5 mixing chambers on all my color enlargers for all smaller negatives with good results. Color paper is quite fast and exposure times have not been excessive, even for very large prints.
The 4x5 mixing chamber will work fine with all formats. The purpose of the smaller mixing chambers is to make the light brighter I think. The light source is very bright and exposure times for smaller formats with the 4x5 chamber are reasonable.
Unless you print Ilfochrome at large magnifications and then get long exposure times that result in reciprocity issues and color shift I would stick with the 4x5 chamber.
I agree with Jerold. I don't have the Beseler enlarger, but I do have an Omega D series enlarger with a similar type of light source. I started out with only the 4x5 mixing chamber and have never had the need to use a mixing chamber designed for smaller formats. Not too long ago I was given a 6x7 mixing chamber. Printing times decreased as expected, but not by that much. I didn't see that as much of an advantage. Printing times are already too short with my lens set to its optimum aperture. Fortunately, my enlarger has a built in attenuator that decreases the light by two stops, giving me enough time for dodging and burning.