Dog wood can be tough to find clean and straight with no twist or too much knots,character,or punky wood...but once found makes a good bow.
Id leave the black walnut alone for now if i were you...its at the bottom of the list of mediocre useable bow woods,and it is not tolerant to novice errors and mistakes...but its such a beautiful wood...i wish it had better properties for bow making.
The best thing youve found and got is the hickory james. Its in the top three woods for someone to learn on. Good clean straight hickory is hard to beat for a novice. Id start there if i were you. Its so tough itll hold up with a less than desired bend..it might take more set with a bad tiller but itll hold,and thats whats important for your first attempts...and hickory is one of the easiest trees to find a good clean nice one...like others have told you once you find one there tends to be more around....any size will basically do as well so small saplings to large trees (but not too big as it could be a lot of work cussing and swearing bringing it down AND splitting it. Get help if its bigger than 8" in diameter to fell and split.