I can see why you'd be a bit worried. Did the place/person you adopted them from tell you what food they were used to eating? You are right that they didn't get plump by eating like birds
My guys are free-fed (which I may be changing 'cause they are getting too plump!) and I've noticed they simply graze frequently during the day. I feed a quality dry food with wet food on occasion.
The little rabbit droppings you describe sounds to me like kitties who might also be a bit constipated
so...my suggestion would be to find out what they've been used to eating for most of their lives, and if these tiny little stools are normal for them. If so, you can discuss with your vet a good way to combat the constipation (there are products to be used weekly or food items you can use such as adding pumpkin to their diet...plain, unspiced).
It *is* important that you get them eating. Unlike us, cats can not live off their fat without becoming very ill (if they do not eat, within a couple of days they can - not will but can - develop a serious illness called hepatic lipidosis (probably mis-spelled that)...so...if you can not get the information on what they ate in recent years, begin trying different foods to tempt their appetite. Even if they are still settling in, they should be eating more than what you describe.
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