I can train anybody to fly using any plane!

That being said, let's qualify the statement. It's going to take somene a hell of a lot longer to learn on a P-51 than a SIG Kadet. But that doesn't mean you can't use a P-51 as a trainer. You want a trainer to have certain attributes that makes the learning process easier and more forgiving of mistakes that newbies make.

Self-righting or positive stability is one of the most desired traits of a trainer followed closely by a light wing loading (not too light though; 14-15oz/sq ft is a good starting point). The trainer should have tricycle gear for good ground tracking on takeoff and landing rollout. And it should have a reliable motor that transitions well from idle to full throttle and doesn't flame out.

I see no problem using a stick as a trainer. If that's what you have then I say go for it. There's no need to spend extra money on a dedicated trainer. I flown many a sitck in my years and have trained a few pilots with them and have had great success. they have a wide speed range and have either nuetral or some positive stability. As long as you set the control throws accordingly they are fine trainers. Plus their wing loadings are relatively light. It takes a bit longer to solo someone with them but that's not really a big issue. A few extra times with the buddy box isn't a factor.

And let's not forget the student's aptitude for R/C. I've seen some guys that couldn't fly worth a damn even if the plane had a FMS Co-pilot installed. Then I've trained guys that could solo after the 2nd or 3rd time out. The individual themself is what determines the rate of progress for a student to progress.

So take that stick, grab a student and go fly.

Jeff