This is true, to a point; however, there are two things to remember here: If the person wants to do a Post ACW impression, then he probably wants to wear a uniform that is Post ACW. The other is that the oft discussed surplus uniforms tended to be classic "Shoddy" in construction, falling apart once in the field ("Melting away in the first good rain" is an often seen quote) as well as made in odd, childlike sizes. Classic wartime contract overrun abuse. Many of these uniforms were given away to friendly tribes, used to clothe women and children!
Thus the Army begged for, and got, a complete redesign and reissue 1872-74. When the 7th went into the field in 76, it had been completely reuniformed, as well as rearmed. Cavalry units in particular were rapidly reissued, as they saw most of the Indian era field work. Infantry units kept the older issue somewhat longer.
All this is academic though, as its simply what Quick Fire wants to do that counts! It must be said that the mid 1870s is about as "anything goes" as it gets for field uniforms! 4 button, 5 button, fireman shirts, jean cloth, bare feet, all can be justified!