I'm curious as to why a round pen panel is not considered safe though? My mare's foaling pen was made of mini sized (4 foot instead of 6 foot tall) panels, and I never had a problem. I did have a problem however, when I put them in a board pen which had panels with no lower board. My mini mule filly got herself lodged under it and scraped up her poor little belly.
OP, you know your jack better than us, and its your choice. I would not risk it myself, but maybe I am too cautious because of the incident with our john, who has NEVER shown any signs of agression until the day he attacked first the colt, then the young man. If you do decide to put them together though, I'd still wait at least a month or two and introduce them very slowly. Don't leave them together even if they look like they're getting along until they've met face-to-face at least a 3-4 times.