(assuming that TP comes after OOT in the timeline)
There is no reason not to assume that. And there are at least two factors that pretty much confirm just that: The Hyrulian character banished to the Twilight Realm that Link has to fight really late in the game (this is me trying to avoid spoilers), and the picture of the fishing pond owner (speculated to be a potential ancestor of the cute girl who runs the fishing pond in TP). And that's just the in-game evidence. An Aounuma interview placed TP about 100 years after OoT and MM.
To get back on topic, while it's certainly possible that Sharp and Flat are in the game, I doubt it. All we know is that SWS happens before OoT (at the beginning of the timeline, to be exact), but even if we assume that Hyrule (at the time of OoT) is just a couple of centuries old, the chance that SWS takes place at exactly those couple of decades where Sharp and Flat lived is still pretty slim. If SWS is too early, then they aren't even born yet, if SWS is too late, then they are already dead.
Oh, and one other thing: If the game truly is about the origin of the Hyrulian coat of arms (as was speculated, if not already confirmed, I'm not sure), then it's probably happening at the very beginning of the royal bloodline, making it even less likely that we'll meet the royal family's court musicians.