One of my 2 current projects is the 6th Ks Cav. I understand what you mean. With the original companies enlisted in 1861 and either mustered out or transferred, I'm looking at approx 2,000 names. The only way I see to do it is a seperate history book and a roster book. The latter would only have the additional info of 5-10 pgs describing the unit's various organizations and possibly a chronology section, especially if you, as the author, desires to add non-military info (not just transcribing names from a list or other documents) to assist the family historian whose ancestor possibly served in the unit (ie, census info, marriages, burials, obits, etc.).

You should at least do a roster book on the 15th Ks, and maybe later do one on the unit's history. There isn't much out there on the Ks units.