On 2 - here's what I do:
I have had starters get a bit too runny when they have gone too long without feeding at room temperature without feeding. When I feed it I roughly double volume of the starter so there's a lot of fresh flour and it doesn't start breaking down the protein.
Ratios - somewhere around 1:1 by weight or 2:1 by volume.
Unless I'm about to bake and building up the starter I throw out half before doubling, so it stays the same size. I also keep the starter really small, a couple heaping tablespoons total, so I'm not using a lot of flour to maintain it. And if I'm not baking in the next few days I put it in the fridge where it can go a couple weeks without feeding.