I have not read all of the prior replies, so hope I am not repeating information.
In terms of shrinkage, it's fine to mix prewashed and non-prewashed fabrics ***as long as you do a normal amount of quilting***. Fabric shrinks differently when washed on its own vs. being washed in a quilt. Quilting ties the fabric to the batting, at which point the batting takes control of how much fabric can shrink. Shrinkage can be a problem if your quilting lines are 10" apart. However, at a more typical 3" or 4" apart, there is enough quilting for the batting to prevent fabrics from shrinking too much.
In terms of bleeding, what I do is take my finished quilt to the local laundromat in order to use a large front-loader with Synthrapol. Synthrapol suspends loose dye particles in water so they do not settle into other fabrics. I use a large front-loader at the laundromat because they hold enough water to dilute dye bleeds. Domestic front-loaders do not use enough water to ensure dilution. Some domestic top-loaders use enough water, but you also need to make sure the washer is big enough for the quilt; there needs to be room for the quilt to move around, so two fabrics are not next too each other for too long at a time. Whatever washer you use, you want to remove the quilt as soon as the cycle is finished, again so that no two fabrics sit next to each other while damp. Synthrapol works best in hot water but, if you do not want to use hot, I would use warm and still expect good results. In my opinion, Synthrapol works better than color catchers. However, you can use color catchers along with Synthrapol. If there is a lot of color in the color catchers when you are done, you will know that you want to use this same process again for the next washing. Color catchers alone are enough if there are just small dye bleeds; however, they cannot handle the amount of dye that can get released from a "bleeder" fabric.