Peas are a fine food! I would suggest you don't leave them in their cage so they don't eat JUST peas and for health reasons. They might stash them in their litterbox, or even pee on them (like my girls do), so you wouldn't want them eating something gross. For me, I feed my girls fresh foods in the evening when they come out for play time. I think the serving size is something like a tablespoon per rat? I don't think you can go too wrong with fresh veggies. I put a bowl in the travel cage on their play area, they eat then play then jump back in to eat more. If there's some left over I put it in their cage. They like to pee in bowls though so I stopped doing that while they did and resumed later. Some people feed their rats fresh stuff throughout the day, or during meals, but I don't think many leave fresh food in their cage just because it'll get dirty fast. Something tough like leaves or a carrot might fare well but meat or potatoes would be pretty gross. Seeing your other post, if your boys aren't coming out then you should give them a bowl of fresh stuff but leave it for a limited time. They eat, you clean up, no half eaten tomatoes stashed for days in the litterbox!