Since I'll wash the flannel (I never pre-wash!! will it shrink a lot??? as in, should I make sure the yardage requirement is well under what amount of fabric I have, just in case??) , should I also pre-wash the cotton fabrics? So that both are pre-shrunk? I like to wait, usually, so the quilt gets that neat crinkly look, but maybe it's better I wash both??? (well, hmmm, the front is already half pieced and not washed, so maybe not an issue.....?)

Rosemary is right. Wash it and dry it so you are sure that it has finished shrinking. Otherwise I have had no problems with it. It seems like your pieces are rather big so I would go for the flannel in all the green parts.
I think it's easy to work with.

Wash the fabric! I have heard that flannel likes to shrink.
Does the flannel like to fray? it might be worth using larger seam allowances.
You could consider starching all the fabrics - I think that I have heard that it will help the flannel to slide whilst quilting on the machine, and that process could help with both the shrinkage & the fraying.

By the way, I haven't actually used flannel myself in quilting, so you might be better to hang on and see if someone who has used it has any better advice.

PS if you treat it carefully I don't see that you would have any trouble with the triangles. Put the flannel on the bottom when piecing a flannel & cotton together. Pin both ends and maybe once in the middle, making sure it is the middle of each piece, and then ease/gently stretch the pieces to fit one another as they go under the machine, if need be.

I am starting on my 2nd son's quilt (pics under the "Legend of Zelda Quilt" label on my blog, linked in my signature) and the back is to be a pieced back as he wants a giant cat on the back. So. I have cotton fabrics for the cat, and for some of the piecing to make it the right size, but the majority of the back is a heavy flannel he picked out.

I thought I'd better ask advice now before I am ready to cut into it

The flannel will be large sections -- here's the EQ7 drawing of the back. Pieced sections are all cotton fabric (regular quilt fabric) and the green background is all flannel.

(well, the drawing shows that the background around the pieced cat is flannel too --- will that be a nightmare, triangles out of flannel joined to triangles out of cotton? Perhaps I can/should sub in a cotton there so that the only flannel used is in the larger background bits?)





Tips? Advice? Cautionary tales? Thanks for any help!