I have a Janome 1600p on a Little Gracie frame. Yes the throat space is limited, but since I have never quilted on a true longarm, I don't know any better☺.
Anyway, it does take some messing with it to get your tension just right with whichever thread your using. I discovered that Glide on the top and Bottomline in the bobbin, doesn't work for me. Stitches just don't want to form. Connecting threads brand of thread works well.
I took my feed dogs out since my machine doesn't drop them.
Just keep trying and you will figure it out. Good luck.