I'm considering the NTG-2, or the NTG-1 + BeachTek DXA-6, with the idea that in the future I may possibly add an Oktava. Any opinions?

Also, I have Sony ECM-23f's which are self powered - can they use phantom power instead? The instructions don't mention phantom. If not, can I assume that the DXA-6 can use these self-powered mics?

(I have a Canon HV-20)

I've bought the NTG2 and I would recommend it.
My reasoning would be that the NTG2 can be powered either
by phantom power or AA batteries.
Real handy if your camera doesn't supply phantom power,
and you do not have a mixer at hand.

I agree. The same mic's, but with the NTG-2 you are able to record without phantom power. The sound with phantom power is slightly better than with the AA-battery btw.

Another reason to go with the NTG-2 is that if you ever get the Sennheiser Evolution G2 wireless system with the plug-on transmitter, you'll be able to have a wireless shotgun.

SKP 100 - no phantom power
SKP 500 - phantom power