The downhill shorts you linked to are padded to protect in the event of a fall, you probably don't need the hip pads and they might make you look fat on your jeans.

The second link is a product that is designed for the sort of use you have in mind, a better choice for you, I think. I've used similar padded underpants and found the padding to be a bit thin, which might fit better in your jeans but doesn't provide the same amount of cushion as most bike shorts. I've purchased baggy style riding shorts with removable padded liners. I use the liners with the shorts they came in and with other pants. If you think you would get use out of the outer shorts as well as the liners, this is a good option. The shorts I have from Zoic came with thicker pads than the pairs I have from Pearl Izumi, both have thicker pads than the padded underwear I have.

Blue jeans are pretty uncomfortable on or off the bike, stiff and heavy with thick seams that are badly positioned for any sort of seated activity. Was that to keep the cowboys from falling asleep in the saddle or just a crude design? Of course, I lived in jeans as a youngster and rode many miles in 'em with normal underwear. I had a Brooks saddle on the bike of my young adulthood. Perhaps something about leather saddles and blue jeans works well together.