After dark gondola ride in Venice

Barb, I cannot recco any specifics but want to laud you for planning to do an after-dark ride--great idea, one we once did ourselves! Nocturnal atmosphere there is truly special. See also taking the very final vaporetto back from Burano at night in order to experience the same nightly wonder.
*You may want post the same question over on Fodors, plus the Italy travel sub-reddit. Also an email to the Venezia Tourism office would surely yield results. Might there also be a Gondolier Association to reach out to?
Tante belle cose.
I am done. The end.

After dark the rate goes up. I have always just found a gondola on a side canal and used them

I was in Venice a few weeks ago. The rates are posted on every gondola departure point. After 9pm the rates go up but it is clearly posted.

On our recent trip to Venice, the native Venetian owners of our B&B gave us suggestions on how to choose a gondolier. Their guidance was very helpful and we had a memorable journey. 1) Stay away from the larger gondolier stands (for example, the one located near Piazza San Marco). 2) Talk to the different gondoliers and be very specific on what you expect for your ride. If you want a gondolier to provide some background on Venice during your ride, then be clear about that before boarding. 3) There is a basic, flat fee for the ride, but you can add extra time if you want. Ask them for the various prices. 4) Find an area of Venice that particularly appeals to you and look for a gondolier there. 5) Look for a gondola that is very clean; that tells you that the gondolier takes pride in their work.
Note: We saw many gondoliers who were on their cell phones while providing rides. If you want a gondolier to be fully present, and not on their phone, then state your expectations up front.

Great advice from all above, Barb!
I am done. The striped gondolier