Does Red RekSai's Accelerate effect apply to any tokens units being created, or does the spell card creating them need to be played from trash? I.e. Death Mark with Flow?

Yes. Rek'Sai, Breacher grants [Accelerate] to any friendly unit played from anywhere other than your hand — and that includes the Shadow Clone token Death Mark creates, whether you play Death Mark from hand or from trash via Flow.

The source of the spell doesn't matter. When Death Mark resolves, its instruction "Play a 0 [S] Shadow Clone unit token" plays that token from a zone other than your hand, so Rek'Sai's static ability gives it [Accelerate] (Rule 805). The token is a single-domain (Shadow) unit, so the cost is [1] Energy + 1 Shadow Power, paid during the Pay Costs step of playing the token — optional. Pay it and it enters ready; skip it and it enters exhausted as usual. (FAQ #11879)