Agree with AdamRice, shorts that are too loose are ok while on the bike.
On a previous trip I had some bike shorts that would slip down when off the bike, I used a nylon strap around the outside of the shorts like a belt to keep them on.
On a bike tour I always wear bike shorts.
You want good visibility. But you will sweat whatever you wear in the Keys. I use a jersey but if you lack a jersey, it is not a deal breaker.
Have a great time. Don't let the long bridge scare you, it is like any other bridge, just takes more time to get across. And you will cross dozens of bridges.
There is a high point on the bridge for a channel for large vessels to pass below the bridge, the photo below is from that high section of the 7 mile bridge.
There is debris on some of the bridges, if you have a rear view mirror so you can see how far back the traffic is, that can be useful to know if you want to veer into the traffic lane to get around the debris.