Buoyancy Shorts

1) Can they be used in a pool swim Triathlon legally?
If wetsuits are not allowed, neither are the shorts. They are neoprene and aid in swimming.

2) If not, am I short-circuiting myself to practice swimming with them when I cannot use them legally?
Depends. If you use them all the time, and you races are non-wetsuit, you will have undermined your training.
If the race will be wetsuit legal for sure, no super big issues.

I see it as a decent swim aid on occasion. It's similar to a swim buoy for drills to allow focus on arms and stroke and kind of space out what the legs are doing and if they are floating or dragging. Used to much and your body doesn't have to work to keep your legs up and without the shorts, your lower half sinks and slows you down.

Moderation would be key.
If you swim 3 times a week, maybe use them for one swim workout.

That being said, I live in Wisconsin, where it is more likely to be Wetsuit-Required than warm water, so I race exclusively in a wetsuit. The short (I have Xterra) are great for pool workouts, without overheating. I use them about 50% of the time, if not more.