To answer a few older posts here.
Although David was the primary employee of Hooper's store (and later owner), there was a series of shows in about 1976 where David was studying to be a lawyer. He was studying at a table placed outside of the store, after Mr. Hooper sent all the kids to go play "on the other side of the street". One day Maria sat down next to him and kept making noise (eating potato chips, playing a radio, tapping her fingernails) that made it difficult for David to study. And for part of this time, David was working nights at Hooper's ("I've GOT to mind the store."). There was this thing where Big Bird kept rigging up devices, like a set of cymbals or a wired record player, to keep David from falling asleep on the job. The whole idea of struggling to work one's way through law school was a good role model.
In another, earlier show, David parenthetically mentioned he went to med school. What happened is Oscar was trying to get the adults to quiet down, so he dressed up as a doctor, complete with a dingy looking white coat and that reflector on this forehead. He then called the adults over and gave some of them a summary diagnosis for some made-up disease, whose only treatment was to lay down in bed in silence. (Susan commented that she was a nurse and would know if she was sick.) David commented that he went to medical school, and noticed Oscar had the "Quiet Mumps(?)", meaning he had listen to everyone playing and having fun!
As for Gordon, I think the reason that they re-cast the character, and twice at that, was because in the earlier days he was kind of a "host" of the show. Note that the Mad Magazine parody "Reality Street" had "Gorgon" (I think that was his name) as the announcer. Later years seemed to treat all adult cast members more or less equally.
The first I ever heard of Susan being a nurse was around the third or fourth season, in which Big Bird packed his bags (including his "spare feather") to move away. When the adults asked him why, he said it was because he didn't "do anything" on Sesame Street
- Mr. Looper -- you own a store
- Susan -- you're a nurse, and you can make sick people feel better
- Luis -- you own a Fix-It shop, so if someone has a broken vacuum cumber you can fix it good as new
The adults responded "Well.... you build nests." Big Bird responded by building an actual-size nest in the middle of the street, and had Mr. Hooper begrudgingly go into it to validate its "importance."
The first time I ever saw Susan in uniform was when they did a split-screen routine of her in uniform and in street clothes. The game was, what was different about them, and what was the same. "We're both named Susan" was one similarity.