RST uses Cheddite hulls, so you have some starting data. Federal Target & Fiocci are, likewise, straight-wall, low-basewad, so the data is interchangeable if you don't get a case of manglumitis.
I'm assuming you are wanting upland & target-type loads, yes? 1200-1250fps & 7/8oz?
Other than fitment (below), 2¾" 20ga <i>IS</i> a 2½" 20ga: assuming the same components, the shorter shell does not build up more pressure than the 2¾".
You'll have two issues loading a short shell: you need a fine(R) grain powder, and you need a short(ER) wad.
- Unique or Universal is about as coarse as you can go, and Longshot, ProReach and 20/28 are finer and less bulky than U or U. (If you have some '20ga powders' on hand, like 800X, Herco, Blue Dot or IMR4756, and want to use them, they will work fine, but, likely, you'll have to add some wad pressure to close anything around or over 18gn.)
- THE wad you'll want to use is the Gualandi G2015. Grafs carries them, and BPI does under their name 'Sporting Short Wad'.
Hodgdon had data for Ched & Fio hulls and G2025 and G/BP20 wads. Ballistically, they are identical to the G2015, just longer.
Call Ballistic Products.. I have seen nothing here that resembles loading data that is tested.. Many of the 2.5" chambered guns are proofed to a lower PSI/LIP than what we see today.. Ballistic products is the leader in the industry for loading short shells.. fully tested data in a proper ballistics lab.. The.. "it looks good to me"..or "I saw no indications of pressure".. just don't cut it..