I went to Clinton, SC and the Ingles supermarket yesterday evening. I decided to look at the lamps and WHOA!  MOST of them were the F96T12s but there were some with T8s and the proper ballast, some with a T8 and a T12 in the same fixture (!!!) and some with T12s in a ballast for T8s ( dim) and some with T8s in a magnetic ballast....bright and erratically flickery. Mostly 3000K and some 4100. The 48 inchers on the end likely are instant start and in one there were 2 daylights. All Sylvania.  The T12s properly ballasted are all close to EOL.  They need to get someone in there and re do the whole lighting system. 

They must have sparky that came from the 240v world, because here it is exactly what is done "properly" (since the T8s here are compatibl with T12s on the same ballasts)

Not here. These are the single pin instant starts.  I think they thought "If they fit in the socket they will work".......................

Well, usually they do, somehow. Only the performance, reliability and life become somewhat questionable...

From that perspective, the instant start F32T8 ballasts are the most robust: They will tolerate virtually any lamp, what fit there, the worst case the light output would be lower. But still I would keep all lamps conected to one ballast the same...