I may be wrong, but I'll try to answer your question. ^^ "chan" is usually added to young children's names, no matter what gender the kid is. It's also used towards girls and young women by friends, family and sometimes teachers at school. Many people add "chan" to pets' names, too. About "kun", I heard it being used by school girls towards their male friends of the same or younger age. Boys may be also adressed like that by teachers, family etc. Many young women call their male friends adding "kun", it's just more sympathetic than not adding any suffix at all when the relatinship is too close for "san". It may be also used towards tomboys. ^ ^

Kun is for younger boys, generally school age, and is addressed by a superior. "Chan" is the one used by friends to each other for any gender.

Arigato gozaimasu Tangoya-san

Yes, they can in all of the cases you mentioned, as long as the people concerned are satisfied with that. And such ways to call others are not uncommon in the real world. An adult man calling an adult woman with 'kun' is a little unusual, but is still possible, for example from a boss to his subordinates in the office.

Do people in adult relationships ever use them in reference to their peers? For example, in reference to adults, if 2 peers are too familiar to use san, can a male add 'kun' to the end of his male friends name? Or can a male add 'chan' to the end of the name of the girl he is dating or even a girl he is just friends with? Or can a woman add 'kun' to the end of the name of a man she is dating or just friends with?