According to Thomas Walter, when the 91st was transferred to the 2nd Division, 3rd Brigade, of the 5th Corps, in May 1863, the other regiments in their brigade 'wore Zouave uniforms'. In April 1864, he indicates 'the brigade commander was desirous that we too should adopt a similar dress, but we would have none of it. We objected to being so easily identified in case we should straggle a little, or be caught on a small foraging expedition of our own. However, we were ordered to drill the zouave tactics and bayonet exercise.' Although he is somewhat dismissive here, he later indicates pride in his company's mastery of the drills: 'Co. A, taken altogether, were not only expert soldiers in company, battalion or zouave drill, ...'. (See Walter: May 1863, April 1864, and June 1864.)
In a letter dated 7 June 1864, Andrew Brown (C) claims that four of the nine regiments in the 91st's brigade were Zouave units--but he doesn't mention which.
A 1933 newspaper article mentioning Joseph Aulabaugh mentions that the 91st was 'a zouave regiment'.
For some information about Zouave uniforms and tactics, and why they were so popular during the Civil War, see http://www.zouave.org/craze.html.