OK I will try to answer these before I have to leave for school.
The plugin does have a readme, but I don't think it's too comprehensive [yet].. (this is about V1, even I have seen little of V2)
I think the plugin should be fine for version 6.1 and the version of Plus! as it works through Plus! (so changes to MSN make no difference to the code required for plugins).
In fact I can guarentee it works: I have just tried it
As far as I know there is no limit to the size that can be played. I know one person who set a 3 min song to a number. That was annoying. If you have a long sound going and you want it to stop you can just play a short one (i suggest /x1). That short one will play, stopping the previous sound.
I have what i believe to be the latest version of Media Player (9.00.00.3008) and sounds play perfectly while that is playing another MP3, WMA, or WMV (the only 3 media files i have laying around my laptop..).
Audiator (V1 at least) can only play MP3 files as far as I know (yup, just tested it with a wma and it said I don't have the file requested - presumably 200.mp3).
I'm not yet sure how the soundpacks will work but I think that as long as the creator has made them properly you can have no problems with them.
I'm now gonna go and get ready for school