I do mostly scrap quilting and often have a number of different thicknesses of fabric. Usually not a problem, sometimes the lighter tight weave of a batik actually holds up better than the thicker traditional calico it is next to, but they aren't causing any wear on each other.
You've already fused the blocks with interfacing that should be enough unless it is very thin, I would probably not worry about it. If you can still clearly see through it then yes, I'd probably use a very light weight muslin or a poly broadcloth bright white.