OK to play 78s with elliptical stylus?

Plato...It's not the shape of the stylus but its size that will degrade the 78's. The stylus tip diameter was considerably larger for 78's, and a LP stylus will scrape along the bottom of the groove instead of riding on the two sides.

Shure makes pickups and stylus to suit the 78 groove dimension, at very reasonable cost. Check their website. (Other manufacturers may also make such pickups).

Plato...Get a cheap equalizer. I'm sure that different brands of 78's used different equalization, just as did different brands of LP's in the early days. The equalizer only needs to make up the difference between the standard RIAA equalization in your preamp and whatever the 78 was made with.

Plato...For really authentic sound, get yourself one of those piezioelectric crystal pickups that used to sell for about $3.50, with genuine osmium stylus. In fact, going further back, there used to be pickups that used triangular cross section wooden (yes) needles, that you would resharpen with a small clipper tool in between records. When I was about ten years old the headmaster of the school that I attended had a state of the art (1948) system which used the wooden needles. Well-behaved students would get to sharpen the needles.

Plato...Not to rain on your parade, but what are you doing for equalization?