Ήταν/Ήτανε can be used interchangeably. Ήμαστε & ήσαστε are a bit obsolete and it's better to only use them in written speech as they sound the same as είμαστε & είσαστε which is the present tense. Not using them at all wouldn't cause you any problems. I wrote them in case someone encounters them in written speech.
- εγώ ήμουν
- εσύ ήσουν
- αυτός, αυτή, αυτό ήταν
- εμείς ήμασταν
- εσείς ήσασταν
- αυτοί, αυτές, αυτά ήταν
είμαι inflects similar to passive verbs, so the -ουν = αυτοί ending from active verbs is a red herring.
The past tense is similar to the imperfect of passive verbs, e.g. βρισκόμουν "I was found", βρισκόσουν "you were found".
The suffix -αν in the Past test is idiomatic and used in many areas, rural mostly, φάγαμαν instead φάγαμε. Τhe form ήσαντε instead of ήταν(ε)/ήσαν(ε) is usual in some areas in Peloponnese. http://www.greek-language.gr/greekLang/modern_greek/studies/dialects/thema_b_5_1/index.html