I have 7 runes, I play Nasus. Can I play Thousand Tailed Watcher for 7, proc Nasus legend and pay accelerate with the 2 runes readied?

No — you can't use Nasus's ready to pay Accelerate for the same Thousand-Tailed Watcher.

Here's why in two sentences. Accelerate is an additional cost you choose to pay during step 4 (Pay costs) of playing the card (Rule 357; FAQ #9414). Nasus's triggered ability fires when the card is fully played (Rule 419.4.a — "when the act of playing the card has been completed") — meaning after Watcher is already on the board and all costs are already paid. By the time Nasus readies 2 runes, the window to pay Accelerate has closed.

Separately, even ignoring the timing issue: with only 7 runes you can generate at most 7 energy by tapping them all (Rule 164.2.a). Watcher with Accelerate costs [8] energy (7 base + 1 Accelerate) plus 2 Mind power. You're 1 energy short. You cannot afford the Accelerate cost from 7 runes alone regardless of the order.

What you can do: Play Thousand-Tailed Watcher for just its base cost [7][M] (enters exhausted). Then Nasus's trigger fires — exhaust Nasus to ready up to 2 runes. You now have 2 ready runes to use for something later in the turn, but Watcher is already on the board exhausted.