I would rehearse every student, then a week or so before select the actual performers for UIL Solo & Ensemble, usually by classification, starting with seniors. Later in the year, after UIL C/SR all of them would perform the piece whenever an opportunity presented itself (Spring Concert, Honors Night, or maybe a community event the entire band wouldn't be able to perform at).
There are lots of WW choir ensembles out there. I've been retired for a few years, however I do remember Helliope Bouquet (sp?) and Maple Leaf Rag by Joplin (don't remember the arrangers) and several Hayden Symphony movements that worked really well. I suggest contacting RBC/JW Pepper.
Many programs do not enter medium ensembles any more, probably because they did not advance to TSSEC at the time, however, these groups benefitted my program by pulling the "family" together resulting in the students listening within their instrumentation and creating a more unified effect all around (same for brass). Also, why not? The percussion always does their larger group ensembles, even though they may not be entered at "Medium Ensembles".