You can feed all sorts of fruits and vegs to different fish in a tank! As you saw with the pleco tank and cucumber, that's a nice easy one for them to eat, and being long, means a few can get onto it and munch happily away.
Most things, like cucumber, best to par-boil then blanch them first, so they're softer and easier for the fish to eat. Just boil the veg for two minutes or so (timing depends on the veg, I only rinse spinach for example) then drain, and plunge the cucumber into bowl of ice cold water/water with ice in it.
Don't be surprised if it takes a few attempts for the fish to recognise this new item to them as food, if they haven't been fed fresh food before. You can get veggie tank clips for this purpose, to hold the food where you want it, or you can weight something down by sticking a fork in it to weigh it down and keep it from floating.
Check out some pleco feeding youtube channels, or do a search on the forum, for the kinds of things you can try! Lots of other fish and inverts like shrimp and snails appreciate a treat like this now and then too.
Yes, uneaten food of any kind can rot and produce ammonia if left in the tank too long. I tend to feed items like that just before bedtime/tank lights going out, then remove the rest by the morning. But how quickly it'll be a risk for ammonia depends on how fast that particular food item breaks down, depends on what it is, how heavily stocked the tank is, if there are a lot of live plants to help with sucking up ammonia etc. Just be sensible and don't leave it to rot in the tank. I tend not to leave anything for longer than about 12 hours, and haven't had a problem.
