One of the things about suspension and the xtracycle, you're changing the balance of the bike, moving your weight more forward. So you're going to have something like a 50/50 split on the wheels rather than about 30/70. The extra load on the front is probably what's causing the mushiness in the suspension fork.
There is more flex with an xtracycle frame so you may be able to get as much cushion as you want with a rigid fork and plush tires. Most xtracyclers I've seen go this route, adding something like a thudbuster if more suspension is needed. You might try something like the Jack Brown's from Riv since you don't have clearance for something wider. I like Vittoria Randonneurs too, pretty durable fast and comfortable.