I've read that peppers will do ok if they each have 1 sq. ft. of area, so if you are staggering between the 18" apart spacings and back a bit, they'll probably be fine. I've planted a single row with 12" spacing and plants were normal size. Shading might be a bit of an issue if they're much smaller than the ones already planted and placed "further back" as you say. Be sure to fertilize them as needed and I think they'll be fine...or you could give them away.