If I am playing nasus legend and I have 7 runes, can I play thousand-tail watcher, then trigger nasus legend, then use the ready runes from the legend to accelerate the watcher?

No. The Accelerate cost has to be paid as part of playing Thousand-Tailed Watcher — before Nasus's "When you play" trigger can even fire — so runes readied by Curator of the Sands come too late (Rules 356.2, 357, 805.2.a).

Here's the order: when you play the Watcher, you declare whether you're paying Accelerate and pay all costs (7 energy + optional 1 energy and 1 Mind power) during the play steps. Only after it enters play do the "when you play" triggers resolve, and that's when Nasus's ability can exhaust itself to ready up to 2 runes (Rule 383). By then the Watcher is already on the board, and Accelerate can't be paid while a unit is in play.

Also worth noting: with exactly 7 runes you'd exhaust all 7 just for the base cost, so you'd have nothing left to cover Accelerate's extra 1 energy + 1 Mind power — even setting the timing aside.