Do those elderly people placed in wheelchairs so they wont fall get ulcers?

What kind of ulcer? Pressure ulcers (decubitus ulcers, "bedsores") occur from pressure which decreases blood flow, often with other factors like poor nutrition, poor hygiene, shear force, inadequate motion (frequent turning or position change every 2 h), wetness, etc. Can be increased in anyone immobilized. Stomach or duodenal ulcers not likely to be increased from being in a wheelchair, without other factors.