The stoma often shrinks down after it heals fully. The surgeon can correct this as it's not part of the valve. A colorectal surgeon should be able to consult and help (they can talk to the KP surgeon).
Using a smaller cath is better then nothing as you need to empty. But, yes, poo takes longer to empty and you really have to watch your diet as the cath holes are tiny.
Have you tried leaving your 30 french taped in attached to a leg beg? Or taped in and use a cath plug on the opened end? Then you just have to remove the cath plug to let poo flow. I used a bit of tape around the plug to hold it in place. I forgot what cath plug it was as it's been years but it worked till I had surgery.
Regarding pouchitis: it can vary. Sometimes diarhea and cramping. Sometimes alot of gas. Sometimes just pain. I thought I had an obstruction but after CTs all was clear. Pain persisted. So, took doxycycline (my old school prefered antibiotic) and it worked.
Try to eat bland for a few days and just see if the pouch is irritated. If problems persist, you may want to try an antibiotic.