It's very individual, and just because a food is fine once, doesn't mean it will be next time. The amount of fiberous foods you're eating may be more significant than the type of food. E.g. you could eat once plum and be fine, but eat lots, or eat one on a day when you've eaten lots of other fruit or veg or other potentially blockage-causing foods, and you may have a problem.
I've had two ileostomies and with both I've tolerated fibre really badly. I can get away with eating maybe one piece of fruit with skin on a day, or one slice of wholemeal bread or whatever, but much more than that leaves me doubled over with stomach cramps and retching every time my stoma has something coming out of it. My current stoma allows me to eat more fibre than my first one, though with both I've learned that I'm particularly sensitive and most people with ileostomies do not have nearly as much trouble as I do.
For me, symptoms do not come after eating the offending food,since they start after that has gone through and blocked the stoma. For me, the symptoms carry on for a few days, usually just when the stoma is active.
Since I'm so sensitive to fibre, I avoid all the foods known to be most likely to cause blockages, like sweetcorn, mushrooms, nuts and seeds, etc.
Try one plum, and don't eat much else that could cause a blockage for a few days before and after.