I would be a little leary of placing too much rotted manure on top of the field if you have pretty deep compaction. It could turn the top layers into too much of a mucky mess. You might consider encouraging deep tap-rooted plants in the pasture to help you break-up that heavy compaction. Dandilions, Queen Anne's lace and a golden rod helped turn one section of my pasture into something less hard than
concrete! And, I made sure to put too much grazing pressure on that part of the field.
Maybe break your field up into smaller sections with temporary tape and use a quadrant system for rotating the ponies. This way, each section of the field will be given some time to recover from the pressure of the herd. If you put in 4 or 5 sections and rotate the ponies at a weekly/week and a half intervals, each field will get at least a months rest between visits. That's pretty good. And, as the fields recover you might be able to increase that time by a day or two.