Greetings rodwha - if you want to reduce the sweetness in your beer, you would start by lowering the mash temperature to about 145-153*F. This will make the beer a little drier, it also reduce the body a bit. If you want to further reduce the sweetness of the beer, you can use a yeast that has a higher level of attenuation - say in the high 70’s or low 80’s.
Regarding your malt selection, I would prefer using the chocolate malt over the black patent. However, if you choose to, you can use both but in small quantities - no more than 2-3oz of the black patent and no more than 3-4oz of the chocolate as a start in a 5G batch.
Hope this helps!
Good luck!