I think the Undersea Cave is missing some maps: Looking in AdvanceMap and there's some maps present, they WERE present in older versions, but they're not here and appear to be the only location of DCRumia.
I cannot find DCRemilia or DCYuuka on any map.
The floor of the ghost tower where you fight your rival has Sweet Scent encounters, but nothing of value...?
DCRumia is found on the bottom-most floor of Mt Moon (same as original) as a VERY rare encounter.
DCRemilia is in Altering Cave, swapping spots with a different dark puppet (I think DCWriggle). I'm actually still trying to find DCYuuka.
I think the original intent was for that to be where you found Youki (as opposed to the typical "rarest goast top floor" every other hack goes with) but then I happened into him the floor just above.