If it were me I wouldn't use that design, there is no means to roto-furl the jib and main sail individually, and this can get you in trouble. There are times on an AI/TI in higher winds where you need to have the main furled in 2 or more turns, or the jib completely furled, and just run off the main, or furl the main completely, and only sail with the jib (extreme wind, using the jib as a storm sail (that's what I do)).
It's also nice to be able to use a halyard line to raise and lower the jib, so you can easily raise and lower the jib separately from the main, as there are times you just done need the jib. Having a halyard, you can also put the jib away, and launch a huge spinnaker if you plan on big downwind runs (spinnaker would be kept in a sail bag when not being used).
Also when the winds are over say 12 mph, you need to be able to furl in both sails a little to balance the load and lessen the weather helm.
If you go to the thread The ultimate Tandem Island ( viewtopic.php?f=69&t=33720
) you can read through and find a really simple furl-able mast topper for your mast, that will help you.
Many guys have added jibs, if you find so posts from Captnchaos he had an AI and did a lot of experimenting with different jibs and concepts and designs, and shares a lot of his hard knocks experience on his posts. Many AI owner have just adapted the standard Hobie Kayak sail to there AI's (20 sq ft), with simple PVC roto-furlers. I think one of the tricks to success is be sure to have a rigid or semi rigid mast under the jib so you have something to furl around, and also because the AI mast is so flexible as it bends under load, the jib can no longer keep it's shape and folds into a U shape (too sloppy to work effectively).
One other minor point is both the AI and the TI have a minor annoying design flaw in the mast holder, that's that 1/4 stud at the bottom of the mast holder, side to side the mast is re-enforced quite well and you shouldn't have any difficulties, but forward to back strain can become a problem once you add the jib. As an example if you were on a strong down wind, and have the mainsail turned to one side and the jib to the other side (batwing configuration), the strain on that little stud is too much and it will break (I have broken quite a few without any re-enforcement), actually you can snap that stud easily just with the boat sitting in your yard, open the main sail all the way, and pull it tight, now give the control line a swift jerk (doesn't have to be too much), the stud will snap every time. The only solutions I know of are to add a rear stay line, which can take some of that load, or re-enforce inside the hull ( I added a small aluminum plate inside my hull) to prevent that stud from breaking. I have never had my boat out without a rear stay line on the mast, the rear stay line is loose 90% of the time, and only comes to play on a downwind. Actually if the 4 1/2 yrs I have had TI's, I have never gone out without my jib, except the first couple month I owned my first TI back in spring 2010, and quickly realized it desperately needed a jib, so I designed and made my own while waiting for Hobie to come out with the promised jib specifically designed for that TI, that never came out (I'm still mad at Hobie over that one, as I wouldn't have bought the boat had I known it didn't have a jib option).
I don't recommend you add side stays, as they really mess up the ability for the main mast to flex and spill wind, rendering the boat pretty useless. I don't recommend you model your jib after what you would normally see on a Hobie cat, those are designed specifically for rigid/stayed masts and don't work out so well on the AI/TI's, you basically have to work around what is already there design wise ( ie... rear stay line, rigid mast for jib, roto-furling separately from the main, and the ability to drop it easily while out on the water, etc, etc , etc).
You can't change the flotation on the AMA's, basically it takes a certain amount of side force (heeling moment) to tip the boat, adding a jib makes it worse (more sail area). Of course you can always hike out on the tramps when running a reach but most people would rather just stay in their seat, so this severely limits what you can do with jibs and spinnakers.
As an example on my TI flying all 265 sq ft of sail and both me and my wife hiked out, we have sailed over 20 mph a few times (obviously the winds were over 20 mph at the time), however in my opinion it's not worth it, it's way to rough, wet, and dangerous, if you want to do that, just get an H16, or a WETA. You will find the jib on a broad reach adds maybe one-two mph to your speed at best. But does allow you to point upwind much higher with more speed (1-2 mph faster with a better VMG), and it also greatly improves the downwind performance, which is not so hot on AI/TI's anyway.
Good Luck, hope this helps you