I wouldn’t worry about using interfacing necessarily (I have made velvet cushions without and it’s been fine). Just clip or pin like mad, I like to use wonder clips (the tiny pegs you referenced) as they don’t leave extra holes. I have a janome and other than for quilting I’m not keen on the walking foot either. I would use the standard foot, lengthen my stitch length a tiny bit (2.6) and if possible on your machine, lower the foot pressure as you are using thicker fabrics. I never alter my tension - I find changing the foot pressure is enough.
In terms of needle I would probably start with a 14 and gauge it from there. Sew slowly!
I really messed up. Why on earth did I opt to make such a big cushion? I must have been crazy. I should have made one of those small, rectanglular cushions. Less fabric to slip
I'd managed to sew the HEMS of the velvet fabric - where the "envelope" pieces meet ( or cross over ). I'd written SEAMS in earlier posts - I'm still really a beginner and I get confused. Anyway, of-course I was still only sewing with velvet - I hadn't even added the "tapestry" fabric piece. I couldn't believe how well the velvet hems sewing turned out. I was thrilled. Gave me a false sense of security.
You're so right about sewing slowly. "Foot Pressure" Well, I put the speed of the foot pedal on "Low". I have a Janome Sewist from a few years back, not the latest model. "foot pressure" I'm not sure I can alter the pressure on the foot. I did have a walking foot but it just didn't work - it's off centre and I should have returned it.
So I tried to sew the velvet pieces to the "tapestry" fabric and the slippage when I got to the end was half a mile wide. I'm too embarrassed to post a picture. Trying to find a solution now. Velvet is lovely but I'm not rolling in it. I should have used common sense and listened to the lady who reminded me I have dogs and velvet ain't too dog-friendly - or vice-versa.