-ㅂ니다 and -아/어요 are both polite sentence endings, so intermixing them is not a problem. And phrases like 감사합니다 and 잘 먹겠습니다 tend to be said in these forms because they are well established set phrases from traditional use (감사합니다 as a greeting and 잘 먹겠습니다 as a semi-ritualistic saying for "I appreciate your preparing the meal"). The informal endings -아/어 and -다 can also be intermixed, but mixing the polite endings with the informal ones is more rare, but even that is not something you never hear, as some adults might talk mixing all the different forms with their friends.