I looked at your tree but there wasn't much on Mattie Bruce. Robert Bruce's father did have a sister named Martha and she could have been called Mattie. The Bruce's were a very large family. I'm sure there's a connection between Mattie and Robert. It looks like they left Kentucky, went to Missouri then on to Fort Scott. I haven't researched them much except for Robert being married to Clemma Watkins. My primary families are Coker and Stinnett.
I'll let you know if I run across anything else that might be of interest to you. I'm in NJ and most of my work is done online but I do get back to Kansas every couple of years.
Joyce