For Red Rek'Sai, does it work for Shadow Clone tokens, like from Death Mark? Can it accelerate?

Yes, you can accelerate Shadow Clone tokens from Death Mark with Rek'Sai, Breacher.

When Death Mark resolves and says "Play a 0 [S] Shadow Clone unit token," that token is played from a zone other than your hand, so Rek'Sai grants it Accelerate. You may pay the optional cost — 1 Energy and 1 Shadow Power — during the Pay Costs step of playing that token. If you do, it enters ready instead of exhausted.

The Shadow Clone has a single domain (Shadow/[S]), so per Rule 805.1.a.1 the Power portion must be Shadow Power — you can't use generic [A] for it. If you choose not to pay, it enters exhausted as normal. (FAQ #11708)