I'll take a stab at your questions with some unofficial answers...
Because individuals in the group have different needs, how much time you spend there outside of the group activity is up to you. So last year some people came down for just the day. We were on the last leg of a motorhome trip. So we got there a day early and left a day late. Some guys rented a canoe the next day and did an extended private trek down the river. But you are basically on your own outside of the actual tour that is being planned.
Gear can range from cutoffs and tenny's on up. I'd say that the preponderance of folks were in the cutoffs and tenny's outfit. The river is wadeable. It runs right through the college campus and is a favorite hangout of college kids in the summer. So if you are not buff, you might want to consider your options. I know with a body like mine, everyone will want to get as much as possible.
Catching fish by net is a fishing license requiring activity. I've only seen the mollies in the protected area of the springs themselves. (along with gar, sunfish, tilapia and turtles) When we were there last we ran into a local professor and some students collecting gambusia in the lower area of the river right by the restaurant. Here
are some pics of our last trip. Sorry, this is really not a very good representation of the whole visit. Maybe someone else has more pics to share.