I have seen some with a barrel nut and some with a barrel nut and lock nut/jam nut. Do the free floaters have to have a jam nut? Will the handguard try to spin off without one? Also Oly arms makes a free floater for a carbine...Will these fit other ARs or are the brand specific? Also if I were to buy an aluminum free floater....can I easily drill a hole and install a sling swivel or picatinny rail?

The Olympic tube should fit any upper that is in spec. and there are many different types of FF rails out there that use diferent mounting meathods, 1 peice that is all in one peice, 2 peice that locks your barrel down then the HG screws on that, 3 peice 1 locks your barrel down a locking nut and then your HG and then you have a few more that I have not used but are 3 peices or more. none should spin off if properly installed your gas tube would also prevent that somewhat.

I have a YankeeHill, but customized it to butt up against the upper without the jamb nut. Giving it a top monorail.  I just ran the barrel nut into the tube, then ran it out 1 turn, then marked it and milled it off there. Theorecially it could rotate off, but the last half turn is pretty hard.  
Not a hi quality pic but you can see it.

Here's a Hogue 2-piece:





Here's an Armalite 3-piece:




Here's a JP 2-piece:




Here's a Hiperform Carbon 1-piece: