Yes it is common to mix 'saya' and 'kamu'.
'Anda' is unnecessary too formal. Rarely use. You can hear politicians in TV using this word to address their opponents formally but impolitely.
To address someone (you):
Anda = (too) formal, polite
Kamu = informal, polite
Kau = informal, impolite
Bapak (for men) / Ibu (for women) = formal, polite
Saudara (for men) / saudari (for women) = formal, polite (I only hear it use in political debates, seminars, churches, formal social gathering)
So, if you're in a working environment or talking to older people, to address them formally and politely, 'bapak (sir) /ibu (ma'am)' would be a better choice. If you're learning Indonesian for business or working purpose, it's safe to address someone using this formula:
Bapak / Pak + (first name)
Ibu / Bu + (first name)
'Anda' would be awkwardly too formal and 'kamu' would be not formal enough.
If you just use Indonesian for travelling or making friends, 'kamu' would be a perfect choice.
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.