If mechs are created from Ferrous Forerunner dying, and then Rek'Sai is played after they appear at base, do the mechs get Accelerate?

No, the Mechs do not get Accelerate. Rek'Sai, Breacher's ability — "Friendly units played from anywhere other than a player's hand have [Accelerate]" — is a static ability that applies as those units are being played. The two Mech tokens from Ferrous Forerunner's Deathknell were already played, resolved, and are sitting on your base before Rek'Sai even enters the board. Accelerate only modifies how a unit enters play ("As you play me, you may pay..."); it cannot be granted retroactively to units already on the battlefield. No rule or FAQ supports a "catch-up" application here.