I think that you have it about right here. There were as Karl remarks many very competent artists of uniforms whose works, although very nice to look at cannot always be instructive as they often do not quote their sources. This is not of course to say that they have no value. Ernest Fort for instance seemed to have spent much of his life copying the work of others - often quite indiscriminately - sometimes using Buquoy artists, who themselves could use distinctly non-contemporary inspirations. He also used his mysteriously disappeared source 'El Guil', the original existance of which would seem to me as being very dubious. I would add Lucien Rousellot to Knotel to a list of artists whose work is carefully documented; not that they are infallible of course as the field of research is not an exact one to say the least.
The last ten years or so, I believe, has been a time when research into military uniforms, in common with other fields, has shown considerable advances. Much contemporary information has become available not least in forums such as this one. My own book, Military Uniforms - the Pictorial Sources (this is not a plug) attempts to fill a gap but I am constantly finding additional information in areas that I rashly thought I had 'cracked'. For instance, the recent postings on Zielinski confirmed that this artist was indeed contemporay - I had been pretty sure that this was the case but lingering doubts remained. Thats what makes the whole thing interesting.