Coming from personal experience with the Grand Exchange, it's a good idea however it isn't really effective for smaller items in a small player community. By small player community I mean less than 50 users. Plus getting rid of shops would increase inflation on basic items. I think keeping shops around would be better. Adding a GE would be cool later on, possibly once the server becomes more active.
The guild idea is interesting though. But there's no point in opening special guilds just for people with 99's in those skills. You said it would be to skill there (I'm guessing without noobs around) and get a cape there.... If they're already 99, they would have a cape. Unless you're talking about making a special cape which would be pointless because they could just go to the guild and get the special cape too.... If you're thinking of adding these guilds to keep noobs from bothering you while you skill, I would recommend creating an area that only people with a certain total level (for example, 1000) or higher are only able to go to. It could be comparable to a donorzone with small areas to skill at inside, bank booths, etc.