If your question is same .. then we got you covered
Answer is YES
REGAMEDLL and REAPI both are compatible with original HLDS
Which means you can run every addon built on REAPI on HLDS as REAPI has many functions which require REGAMEDLL and now you know .. REGAMEDLL is compatible with HLDS too !
REUNION is not a addon for REGAMEDLL ( Many users dont know this )
Reunion requires REHLDS to run !!