I am not sure if my experience in Davis County would hold true for every other county in Iowa, to be sure you should check with the office in your county. For me, I stop by the NRCS office every spring and update a map of my farm that is there on file with the location of the food plots I will be putting in that year. Their main concern is that the location(s) be on flat, non-HEL ground.
But pretty much anything is acceptable in the 10% area of the CRP acres, brassicas and clover are just fine. In fact, you may be able to plant a clover mix in acres beyond the 10% limit. The NRCS office may well have up-to-date aerial photos of your place too, so I would recommend adhering to the 10% limitation, BUT I have never found them to be unreasonable either and I am pretty sure the "CRP cops" aren't going to show up if you have a bit more than the 10%.
As far as what to plant...I would read Dbltree's forum and follow his advice.