It is acceptable to use the formal 'saya' and informal 'kamu' in the same sentance? Do Indonesians mix them in routine conversations/writings? Should this be either entirely formal (saya and Anda) or entirely informal (aku and kamu)?
My wife tells me that they would often mix these ... that the rules about formal and informal are not so important with common words. The place a grew term emphasis on who you are talking to; hence would use 'anda' when talking to someone they do not know. When talking among friends or aquaintances, formal and informal are often mixed.