:eek So many questions!
This should probably be placed in Molded Armor discussion...
1. Yes there are different types/different manufacturers of body filler. If I knew more about what you are trying to do with it I could make a specific recommendation.
Polyester Resin: Liquid... brush it on. Should be used with fiberglass cloth, applied in layers.
Body Filler (Bondo) Thick, creamy gap filler. Use a plastic spatula or putty knife to apply.
Fiberglass Strand Filler. Resin with the fiberglass mixed in. Mixes like taffy, hard to apply. Hardens like a rock. Not advisable to use for most armor applications..
2. If you're talking about the fiberglass strand filler ... the stuff that has pre-chopped strand already mixed in, then I guess no. What you want is Fiberglass Resin... it's a liquid... so easier to apply. You may be working with the wrong stuff, I suspect.
3. Of course you can sand Fiberglass .... in any form. Use low grit sandpaper to start, like 60 grit. Also a handheld sander helps alot.
4. Polyester (Fiberglass) resin is your best bet. Remeber... not the stuff with the fiberglass mixed in... that stuff is like taffy and near impossible to apply neatly to your armor.
5. I suppose you could use epoxy putty... the kind that comes in stick form that you mix by hand, but it's gonna be killer expensive. Theres also spot glazing filler... the bright red stuff. It sands easy... can only be built up in 1/8" layers.. isn't structurally strong... shrinks as it dries, and again crazy expensive.
6. Silicone.... what for filling gaps? Nope.... you can't sand silicone... it's a flexible rubber.
7. You're not a noob. You can't learn how to do this stuff without asking questions. Just be sure to do a good deal of research and testing before you start slathering the wrong material over your paper patterns. Proper research and experimentation will yield you better results than jumping in with both feet before you know what you are doing.
Hope that helps... also check these tutorials out if you need more instruction with fiberglassing:
http://www.paper-wings.org/tutorials/armor/fiberglass.html
http://www.paper-wings.org/tutorials/armor/bondo.html
erm... they were available in the TUTORIAL LINKS thread.... *Cough * read* Cough*