We are using 20mm series of t-slot aluminum. To attach tetrix pieces to them, we put a 6-32 keps nut (used in tetrix system) in the t-slot - it just fits (wth some effort). After this, we attach the tetrix pieces using regular 6-32 screws. When we need a stronger connnection, we just drill out the hole in tetrix piece and use the metric (m5) screw with the t-slot nut to attach.
However, most of our robot uis built out of t-slot profile; we do not use that many tetrix pieces.
FYI: adafruit also sells 15-mm t-slot profile:
http://www.adafruit.com/products/1275
Haven't tried it myself yet