I talked to a couple guys at the Big Iron Farm Show that were using 12 row 20" heads on 9610's. They were using the Geringhoff rota disc but had put on direct drives. The one said he was switching to 30" rows and was going with a 16 row Geringhoff on a new 9860 this year. Haven't talked to him again to see how he liked it. I know the rota disc takes a little more power than a conventional so i'd think you'd be fine with a conventional knife roll head.
We run a 12 row on a 9650 walker. It handles it fine, I would think a 9600 would be about the same, maybe a little less horsepower. Thing there is the weight of the head, we loaded the rear tires. Seen lots of pics of the 9600 with 12 row heads though.
There used to be lots of 12 rows on 9600 and 9610 around here before the STS came. The feederhouse reverser is a weak spot and the clean grain elevator is a bottle neck. The 9650 is improved on both issues.