The choice of float line or reel depends a lot on where you dive and what you shoot, and I just spent two more days on Spearboard discussing it, so I'm burnt out for now.
So I'll evade that part of the question and answer the second part- yes, you can switch back and forth, although its easiest if you have two different shafts, one rigged for breakaway and the other for a reel.
I use these treaded stainless steel inserts from West Marine in all my guns, even those from which I don't plan to take the reel on and off. I think they take 10-24 or 10-32 machine screws.
You drill a pilot hole in the gun, screw in the insert, and then use machine screws to attach the reel. That way you aren't taking wood screws in and out.
In addition to making it easier to switch from reel to float line, they make it easier to remove the reel for travel.