I have personally built two trout rods from fly blanks. A matched set, one for me and one for the wife...

I used the St. Croix SCIV 7'6" fly rod blank... I believe it is a 3 or 4 weight... They are absolutely awsome... teamed up with the smallest shimano 500 I could find...

a 6" trout will rip your face off!!! <img border="0" alt="[eat]" title="" src="graemlins/eat.gif" />

I've landed trout as big as 5 lbs... whoo hooo!

I wouldn't go to much lighter than 3 or 4 weight with the St Croix blanks. The tip dia is pretty small as it is. plus, the single foot spin guides are quite a bit heavier the the traditional fly snake guides so the rod will soften up quite a bit.