Im looking to get my first used longtail . Ive got a 20" transom. My question is. Are long tails universal transom or do i have to get one for a tall transom?

Have to get one for a tall transom....

I was reading on a different post and it said I could bend the cavitavion plate?? Does that work?

that is for fine tuning. but you have to figure out if the motor you purchase is for tall or short its pretty black and white . when it comes to tall or short transom motors

Get your rig first, should be tall transom. Try it out on your boat , run it , you should bend it up or down in small 1/8 th inch increments , to adjust how it rides. Your plate should ride just below the surface while under power without it wanting to jump out of the water, with my rig it finds it's own sweet spot . While I wouldn't recommend it I can let go of the handle with both hands and it stays perfectly in place( doesn't jump around) hope this helps.

on a straight away i can set the throttle and run no handed all the time (not recomended)

You will need to find a motor matched to the transom height. You dont want to put a short transom motor on a tall transom boat. It will fight you the entire time and you will hate it.