At 18 st over 4 inches the gauge is generally a worsted weight yarn. When substituting yarns you want to look for a yarn that is appx the same gauge. Then you can swatch and see with different needle sizes if you can get the same gauge.
As for yarn characteristics.. bamboo is usually drapey from what I've read. Knitting at a tighter gauge can sometimes help this. Maybe not the time to test the theory with your first sweater though. The BS cotton fleece looks like it could work, but you'd need to swatch.
Don't forget to check the comments for any yarn you are considering, too. They can tell a lot.