Depending on the cause of the short nap (if you can identify it), my personal rule of thumb is:
UT=no point in resettling bc LO slept long enough to feel refreshed after doing a short A time
OT=would try to extend for ~20-30 minutes
In either case, if LO wouldn't go back to sleep, I just got him up, fed him close to the E time, shortened the A time (which started from the moment LO woke up early from nap), and then put down. They normally don't make up the time they didn't sleep during the previous nap...which is something I kept expecting to happen when DS was younger so that our routine would stay on track. Once I learned that, then I realized that I just had to readjust the following A time (after the short nap) to help LO have a long, restorative nap so that he wouldn't get OT.
DS is 9 wo and I seem to be getting into a 2.5 hr feed routine where as before it was 3 hr.. is this OK? I don't really want to keep him up to make the 3 hr mark and risk OT.
No, don't keep him up the extra 30 minutes. You'll need to follow his cues and get him to bed within his sleep window so that he doesn't end up OT. Overall, EASY is a routine--a pattern--so don't worry about the exact times.
If you'd like help with the short naps, you can post what your LO's day looked like and we can take a look
HTH a bit!