Don't understand about the skintone comment. Why is it difficult to achieve in that setup?Â
Yes, any FX lens can mount on the D90.
The Nikon F mount has never changed since 1959. You can still mount old lenses on your latest Nikon bodies unlike the Canon.
Like Andy214 does not know?Â

I thought the challenge was to get
ambient as well as subject illuminated in the same WB? Not flash as the main light.
What has gels got to do with it? I guess you did not read.
There was never a discussion of have or don't have gel. Of course when you don't have a gel, you make use of the normal speedlight like the way it is supposed to be used. How else is everybody using their speedlights?Â
That's correct. That was what I was trying to put forward. For skintones, I never like to put high contrast or high saturation.
Not too sure of your problem but metering can be fooled and the flash will underexpose. Perhaps that is why you see suggestions to use centre weight metering and using full manual flash or full manual exposure to fix more variables and less chances of the camera screwing up.
For me, the advantage of a f/2.8 lens is that I can expose close to the ambient light and let the flash be the fill light.
I rest my case, you just don't get what I'm trying to tell here.
No point trying to explain to you.
OT: alpha_company, I wonder why... my NOD32 blocks your signature ._. first time coming across this kinda condition lol.