Yes, a 15-90 will drop the feed dogs, and that is the best way to do this. There are some older 15's that don't drop the feed dogs
As to the 15-90 or 91, the only difference is the power source, the 90 has an exterior motor and the 91 has the built on potted motor - but the rest is the same so the feed dogs drop on both.
And you don't really need to put the feed dogs to Zero on the stitch length, I never bother with that as I leave mine set up for FMQ and winding bobbins for my Long Arm.
I did quite a bit of FMQ on a 15-90 that started out as a treadle machine. We put the motor on it and gifted it to a 12 year old had been given a brand new machine as a gift, but found it could not handle anything heavy - and she wanted to make a jeans quilt.