When I get pieces of comb that aren't tall enough to reach top to bottom in a frame, I don't use them. However, I've seen some pictures from other peoples' cut-outs that show a piece of comb banded in, resting on the bottom bar, with a large gap between the top bar and the top of the comb. What happens with these? Do the bees actually build comb upward and attach the comb to the top? I've never had any luck getting bees to build from the bottom up.
I've also seen pics of frames with starter strips and cut out comb. Do the starter strips make a difference? So far, I've found that if the comb reaches the top bar, they will quickly fix it in place and expand to either side.