ultraply & underlayment staple question

I plan to install UltraplyXL 1/4" underlayment over a plywood subfloor. The manufacturer recommends staple with 7/8" leg & 3/8" crown. Searching for an electric staple gun that handles this staple has yielded one cordless tool which is unavailable and would cost close to $500.00. My questions are: Can I move up to a 1/2" crown staple? Will a 9/16" leg staple be long enough? I'm a do-it-yourselfer so would prefer not to spend a lot for a tool I will use once. Thank you,