I have studied the Donelson Campaign for a number of years and lead tours there all the time (including today).

The Order of Battle for John Floyd's VA Brigade is as follows:

20th Mississippi Infantry
36th Virginia Infantry
50th Virginia Infantry
51st Virginia Infantry
56th Virginia Infantry

Goochland Artillery; French's VA Battery, Jackson's VA Battery

The regiments of the brigade were split into three brigades upon arrival between Wharton, McCausland and Baldwin.

I do not know if the 51st VA Militia morphed into the 51st Virginia Infantry but there is no VA unit with a militia designation in the Donelson OOB. Lee Wallace's excellent reference book on VA units might have the details.

Greg Biggs