Hi-
The opacity of the GIFTI surfaces can be toggled through (4 total settings) by hitting the letter 'o' while the mouse is over the SUMA viewer.
The way that 'lower opacity' is achieved in SUMA is through a pixellated mesh that becomes less dense.
Are you wanting to have a whole brain's anatomical surface viewed simultaneously with tracts? Or are these sparse regions of a network that you've tracted among and want to view simultaneously? In the first case, the anatomy may be more easily represented just a slices of a volume; in the latter case, are a huge amount of the tracts obfuscated?
--pt