Yes you can litter train cats who are used to going anywhere outside. Just make sure to show them where the litter box is, make sure it is easily accessable, and clean as possible at all times! A rule of thumb is one litter box per cat, but we've used just one box for two cats before without problems. You just need to clean it more often.

Indoor cats, I'm not sure if they EAT less because outdoor cats may be more active persay but indoor cats still need their exercise, too. So on that issue I am not entirely sure. They may eat less because they WILL be less active. Its just a trial and error thing you will have to work out what is best for your kitties

You can keep them from scratching furniture by buying scratching posts or those cardboard scratching things and put catnip on them. If your cats are adament to using your couch to scratch on, they do sell guards you can place on corners of couches and such that will protect them. Worst case scenario there are sprays you can buy that will keep them away from areas you don't want them to be or scratch at.