mostly those are just cute ways of naming ppl, but there might be some meaning behind them as well, alongside of intimacy and feelings of ppl using those short names for others, Japanese ppl take it very seriously who is calling them how, for example, a person that doesn't like someone wil lnever allow that person to call him/her by first name, and most probably will force to use -san/-senpai etc as honorific.
as for Kanon, you might first go watch classic Kanon and then 2006, but halfway go grab tissues ;]