The companies in the 7th Cavalry on June 25 averaged 66. This is using the information in the back of the 1995 edition of Men With Custer by Ken Hammer. However, going into battle, the companies had about 40, due to detachments with the pack train, and those left behind at Powder River for a variety of reasons.
Examining the rosters in Vaughn's book With Crook at the Rosebud, the five companies of infantry averaged 40 as well.
The 15 cavalry companies on the Rosebud campaign averaged 55.6.
In the case of the figures for the Rosebud, I do not know if these included men not actually present for battle.
For my wargame units I use two platoons of 10 men per company, plus a company commander, trumpeter and orderly.
You asked about regiments. The entire 7th Cavalry was together in the field, all twelve companies. That was not common.
Crook's five infantry companies at the Rosebud were from two different regiments, the 4th and the 9th.
His fifteen cavalry companies were from two different regiments as well, the 2nd and the 3rd.
I hope this helps.
Tom