as long as you are using a quality brand of floss you will not experience any color running from your floss...it is color-fast;
i usually use muslin for red-work blocks, you can either get bleached or natural, up to you if you want a creamy & red quilt or if you want a stark white & red quilt.
if your designs are around 8" i would start with 11"or 12" squares to embroider on then trim/square up to 10 1/2" after the embroidery is complete. if you want the finished blocks larger than 10" cut them an inch or two larger than the finished size wanted and trim them after stitching.
when i made my first red-work quilt i spent almost a year picking up different red prints to put it together when i got the blocks all embroidered, it was lots of fun! mine is our favorite picnic blanket...the patterns were called ("love beneith the willows") and is animated dishes, so cute :thumbup: and i found vintage fabrics to put it together....i'm working on putting together a 'blue-work' quilt now :thumbup: