get a piece of slate for a base. drill a hole through the slate, then screw the slate to the wood. you can now take the piece and put it in the tank, and push the slate down into the substrate if you dont want to see it. it wont float.
should be okay, with guppies, you just want more females than males. as for the fry, they might get eaten, but id not worry too much. once you start having guppy babies, you will very soon have too many babies.