I would disagree on the following grounds:
- Models (ICs) that join a unit become part of that unit for all respects. The combined models become a single unit.
- The Eldar psychic powers clearly state that you select a friendly eldar unit as your target.
- A unit joined by a non-eldar model (read, anything not within the eldar codex) is no longer an eldar unit. It is a unit composed of eldar and other models. This would therefore not meet the criteria set out in the eldar powers rules.
- Because the combined unit is no longer an "eldar" unit (it is a mixed one) you cannot select the unit as the target for the power.
Note, this would not necessarily extend to the "generic" powers in the rule book as they not differentiate between races but simply friendly or enemy models / units. It is also worth noting that there are some powers there that are not only similar in rules to the eldar powers, but also... better. Can anyone say 12" range Guide?
On the point about whether or not "dark eldar" models count as eldar then this is a no. simply put dark eldar are dark eldar, eldar are eldar. Exactly how space marines are space marines, whilst dark angels are dark angels.
yes, both dark angels and dark eldar might be "types" of eldar and space marines... but they are, in game mechanics not. One should not be confused between fluff and rules terminology.