I am probably Don's worst pain here
but as for me, I'm trying to avoid unofficial modifications, one-offs, casual pick-ups and paper projects.
On the other hand, sometimes vehicles built/modified in a short series could have an impact, that makes them worth to be added (like German Dicker Max)
I, for one, suggested last year to add 6 Carden-Loyd Mk.VI tankettes to Belgium, bought in 1932, which were their only tanks apart from FT in early period. By 1934, they were converted to tank destroyers, and were in service by the time of the German invasion, so they might have destroyed something (Polish #702 TK-SD can serve as a pattern, with 2-men crew)