Hey,
1.You can change the proportions of the skeleton. Just Can't add new bones, yet.
2.No, the bones have the same name as the default SL ones so it works fine. Yesterday as a test I made a rig with freakishly long arms and short legs and it worked fine.
3.Load the fbxmaya.mll plugin and youll be able to export dae files. Probably a better option is exporting it as a fbx and then convert it with autodesk fbx 2011. (Maya 2012's collada is based on a later version than SL's)