I have the exact numbers for the Batlte of the Little Bighorn somewhere.
Essentially there are 12 Companies arranged into three squadrons of four. Custer split this up unevenly into the groups shown in the first picture, he took 5 companies and left Reno and Benteen with three each. Company B guarded the supply train.
There were about 50-60 men in each company at the Little Bighorn, some had only 40 or so. each Company had identical coloured horses (like Napoleon did).
Reports differ as to what colours were where but everyone agrees that E Coy had the greys (white) horses, even the Indians commented on them.
The cavalry dismounted and fought on foot, they are really mounted infantry, three men formed a fire group and the 4th took the horses to the rear. Often the Indians would try to attack the horse holders because a cavalryman without his horse couldn't escape.
Custer also took the regimental band with him on campaign, they would play 'Garry Owen' to signal dawn attacks. I was going to convert some figures into a band but never got round to it.
Incidently, and this is not a comment on your excellent painting, but only officers and ncos had the yellow stripe on their legs, enlisted men didn't. Then again, they didn't have yellow neckerchiefs and all of mine do.
You have to have some concession to Hollywood. :

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If you need more detail let me know and I'll try to find my list. The Osprey Little Bighorn book is one of the best on the subject and is well worth getting. Sadly Son of the Morning Star is not available on dvd, it's a very accurate portrayal of the equipment and tactics used by the cavalry.