I think it's a good idea not to deviate too far-- even with doubling the thinner stuff, you're talking about a difference of about 16% of length per gram. But having said that, thickness doesn't always equal weight-- some yarns will have a the same thickness but due to the spinning process, will be of different weights. If the weight and/or thickness are very off, the density an drape of the fabric can come out very differently. But I always substitute yarns for various reasons.
So, as Jan recommended, given that plus the different tensions with which everyone knits, make a swatch and see what the gauge comes to. I would make an extra large swatch to see if the drape is okay, as well.
Ann Budd's Knitters' Handy Book of Patterns gives yardage as well as weight for various projects, and she also has a separate little laminated leaflet with yardages in it: http://www.interweavestore.com/store/p/1693-The-Knitter-s-Handy-Guide-To-Yarn-Requirements.aspx.