And again: welcome. I'll echo some of the things already said.
What grain is important? It really doesn't matter when you're backing with bamboo. Grain orientation and violation are important factors when making a selfbow, or selecting a board to use for backings. When applying a backing of bamboo, you're adding a layer of continuous, un-violated fibers to the back's surface.
Can you back a bow with bamboo flooring? No. The amount of power fibers at the surface is too small. Plus, there's no telling if the fibers are parallel with the surface of the board anyway. It makes for a good core or belly material though.
What length is good? I'd say 68-72 inches from nock to nock for a first (and even fifteenth) effort. It gives you lots of wood to spread your tillering across, and will allow you to shorten it some if the weight comes in too low.
What tools do you need? At a bare minimum?
http://tharwavalleyforge.com/articles/t ... re-minimum
I suspect a scroll saw will not have the grunt or capacity to really contribute much to bowmaking.
What glue? I've used Titebond 3 with good success, but the worldwide standard for laminating bows seems to be Smooth-On EA-40. I think it's terrific and it's about the same price as Techniglue, or so it seems. Two pints (one pint each of resin and hardener) will easily do 12 backed bows.
Here's a buildalong I did many years ago:
http://www.ozbow.net/phpBB3/viewtopic.php?f=3&t=4295
Please, please, please show us your progress and ask for help along the way.