I own 2 different baggy suits, a BFI and a BARE and for what it is worth, if I had to do it over I would buy a tighter fitting one for slalom. The issue becomes trapped air and if you take a good fall, the air has to escape so it will blow out thru the seal and you get wet. That being said, if you make certain to keep the seals higher on your calf (not around the ankle) you will get a better/tighter fit.
As for sharing, while it sounds like a good idea, make sure you have an agreement for who will be paying to replace the seals when they are ripped because it is not a matter of if rather when. The seals are very tight when you first get any drysuit and they stretch over time. As you are putting it off and on, expecially in cold weather where you tend to lose some dexterity, it is very easy to rip a seal. The BFI seals are longer, really tight and fit great but also tend to be more expensive to replace due to the neoprene overlay. If you are not going to be footing in it, scratch the BFI from your list altogether.
Just a tip on keeping your seals in good shape....treat them with 303 protectant and keep them out of the sunlight. When the suit is not in use and is dry, store it wrapped up in a sealed plastic bag. The 303 will also help you slide in and out of the seals when the time comes.
Good luck and hope this helps.