Birddog,
I would not see a problem with adding some brown trout. In fact variety is the spice of life. I have had brooks, browns, and rainbows in the same pond. The only disadvantage I would see with browns is they are more difficult to harvest. As long as your fish sizes are similar I would see no problems.
Browns will also tolerate more marginal water conditions if by some chance your pond gets warmer than you realized your browns are more likely to make it.
