A smaller old $1000 baler takes about 17 hp to run it - to charge up the flywheel.
It takes another 5-10 hp to move it and the tractor through the field.
A 20 hp tractor will sorta run the baler, but it won't be fun. Well underpowered.
Two problems beyond that:
Tail waging the doge - the baler will wiegh more than the tractor, and will eaily get away on you going down a hill. Dangerous. As well the back & foth plunger forces are going to make it a rocking good ride. Baler often has negative tongue weight, meaning it lifts up on the rear of your tractor, making braes or traction even less effective....
And, the driveline on a under 30 hp compact tractor is _not_ designed to deal with a 40 hp flywheel on a baler. That big flywheel on the baler stores power, perhaps 40 hp or so, and the flywheel actually does the heavy pushing. It only takes a little less than 1/2 the hp to charge up the flywheel, but you are truly putting a 20 hp tractor on a 40 hp baler, and it will mash up the pto parts of your tractor in a while.
All around, it would be the tail wagging the dog. If I had 5 acres of hay laying down, my other tractors died, sure I'd hook up the 20 hp compact to the old baler and finish the field before that rain cloud came. It would be possible.
But it would not be fun, it would not be productive, and it would not be good to _plan_ to do this all the time.
My opinion. 30 hp bare min to do a good safe nice job of baling with an older small square baler, and better have a heavier weighted tractor to control the baler as you drive - fluid, wheel weights, something.
--->Paul