Edison used round perfs, but they caused problems making prints particularly when the (Nitrate) film shurnk. Bell and Howell made a perf with rounded ends and stright sides, (now called BH perf or N (Negative) perf, but the sharp transitions would sometimes crack at high speeds. Kodak made the perfs still photographers are familar with to avoid this wih all sides stright and rounded corners. The "height" of the hole is a bit higher however. These are called KS (Kodak Standrd) or P (positive).
Just to allow for the fact that more modern films shrink less than the nitrate ones did, it was found useful to space the Negative perfs a bit (4 ten thousands) closer than the positive perfs to avoid slippage in the printer. so you generaly find Movie negative with a "BH-1866" perf while positive print film is supplied as KS-1870, as is most film inteded for still camera use. I have never heard of a still camera that is bothered by the difference, and since still cameras one use the perfs once, the risk of the BH perf causing a rip is rather remote. A movie camera would become very noisy if trying to use the wrong perf pitch.
there have been other specs, the Duvray-Howell DH (like a Kodak but with a bell and howell hight, CS (cinemascope- AKA Fox Holes) with smaller perfs to allow room for magnetic soundtracks.
As for why the 100 ft rolls are not standrd items any more, remeber that 35mm Movie film runs at 90 feet a minute, so a 100 ft load is really only useful for some special applications like a "crash Camera" that is mounted in harms way. They used to rig up eyemo cameras which take a 100 ft spool for car crashes and the like. If you order a lot of film Kodak will "finish to order" a lot of stocks but the price and Minimum order is a lot higher then for warehouse items.
If you can find some friends that want to split a roll, some of the local movie labs will re-spool film to smaller rolls for a few bucks a roll. if you can't supply empty cans they will probaly want a couple more bucks for a can/bag/core combination. They are most likly to use a 2 inch core. If you say that you are not using the editing numbers they don't have to rewind the rolls to put them "start end out" so that might save them a few minutes.