keep shots off your actual flouro leader. use the line below the float for shots, with the swivel as your last "weight"
some guys use a swivel right under the float to have a shot line but thats a personal preference thing.
i personally leave a length of my mainline below the float and slide the float up/down depending on what depth i need before i set out from the car.
12inch-2foot flouro leader depending on water clarity after that
a shotline is handy though if you go to several different bodies of water, you can have several depths/current patterns ready to go. i just dont find it usefull personally. i carry very little when i go and the less rolls of ready mades i have to carry the better.
as for knowing if you have enough weight bud. as long as the float stays upright and you dont hit bottom too often youre good. if you never hit bottom at all you might consider increasing the length a bit. typically you should ding the odd rock on the bottom but not be dragging. if you are dragging youll notice your float tilting downstream a lot or moving slower then the current. just try to observe that your leader floats ahead of the float.
if not you might need to put more weight near the float and less near the leader.
for length of your whole setup, going by water speeds, usually 2x the depth from float to bait is a good place to start. faster water=longer. slower=shorter
check out this link
http://www.anglersinternational.com/Shotting%20Patterns.pdf for pretty much everything you need to know about setting up your shots