I think you'll be fine with most string, just avoid waxy string. I also don't like the way things seem to work out with fishing line. It's a fine material, but it never breaks down and I always end up wishing I used thread.
As far as the fern debate, My java ferns do ok planted in the substrate, but I notice faster growth when the roots are free to feed off the watercolumn. I like how it encourages them to grow so much root mass. I like to tie them to wood in places where the roots can drape over and around, and eventually reach the substrate. But this makes moving them harder.
Putting the floaters outside or in the window ought to be fine, although insects love to eat floating plants. Smaller stuff like duckweed will grow insanely quick in sun. Personally, I like the cover floaters give my smaller fish, but I don't keep them in all my tanks, because they take over fairly quickly. The red root floaters seem to be the slowest growers I have had, and don't like being disturbed much... but with some bright light and time, the roots come to look really nice. They need very little to go on once they're a full size plant... I gave some to my GF months ago, which she keeps in a glass bowl on the windowsill, watered with tap water. It looks as nice as the nicest in my tank, and the bowl doesn't have a spec of algae.