You have to write the mask groups in lowercase
Code:
$MscSolid => $mscsolid
Don't ask me why, but it works.
precomp is always used by xtool (well, the command precomp, not the tool). You can't call xtool without precomp command (precomp = precompression). That's what xtool is for. You can only change used codecs or options.
For uncompressable files use $void mask group (or your own). Open arc.groups with a text editor and add files to $void mask (by filename (+ subfolder) or by file extension (*.abc). You can define files for every other mask group or make a new group ($groupxyz).