Gravel? I should of thought of that! You are very smart...
But I read that worms dont like rocks. They can't eat them and moss will start growing on the gravel because moss is the only common plant that can grow on rocks.
Dont forget to drill holes in the sides too, and the top.
For a starter batch, do you mean for worms or for bedding? If for worms, I would go reds, but I have 2 worm farms, one crawlers and the others red.
They breed much faster than crawlers and you can have more in a smaller bin. For bedding, coconut fibers, newpaper/cardboard, and/or brown leaves. Add a little top soil too. They need the grit to digest their food.
Happy farmin'