Ahoj!
As to the ethnic composition of those regiments - I simply know where these towns are, and I know the crazy quilt pattern of nationalities which lived there before WWII. There were islands and peninsulas of Polish speakers separating the Belarus and Lithuanian ethnic areas which ran from Belostok to Dyneburg (Daugavpils). And scattered Polish minorities (mostly urban) all over the place. The Tatars were also all over the place, but in villages.
Kobryn - a small town just to the east of Brest Litovsk. At that time the locals had no sense of nationality, they were the "local people". However, they certainly were not Polish. Today it is solidily Belarus area, with even Ucranian dialect speakers considering themselves to be Belarus. So I say the regiment is Belarus with a sprinkling of Ucranians
Molodeczno - this is IIRC near the edge of Polish and Belarus speaking areas, so I imagine the regiment to be mixed, maybe 50-50, or with slight majority of Belarus.
Lida - this is a big Polish "island" between Belarus and Lithuanian areas. I suspect this to be a mostly Polish regiment, with a Belarus minority, and possibly Lithuanian too (but much smaller than the Belarus component) - this depends how much to the east or west of the town did the regiment recruit.
CORRECTION:
Going by memory, I misplaced Lida (somehow I always put it to the north, not south of Wilno). Lithuanians are rather unlikely in it, a Polish-Belarus unit.
Nowe-Troki (Trakai - if you are using a Lithuanian map) - I know the least about it. I guess it is to the west of Wilno, so it should be Lithuanian. Some sort of Polish minority cannot be ruled out.
CORRECTION:
I checked the map, and it is to the west of Wilno. But very close (Wilno was on a big Polish "island", which also included Lida to the south), so it could be mostly Polish, but there should be some sort of Lithuanian element in it.
ADDED LATER:
And now for some fellow Central European to pounce on me

For those who would like to see where these towns - not exactly household names - are, check the excellent map at the warchron site:
http://www.warchron.com/imagePages/maps/german/2a/2b.htmTroki
http://www.warchron.com/imagePages/maps/german/3a/3b.htmKobryn, Lida
And I forgot about the Grodno gubernia - sorry.
Borys