darrell:
Yes, you can add HSB to your pond. If your biggest LMB are 13", 6"-7" HSB would be susceptible to predation. I would want to put in at least 8"-9" HSB if I were you.
Some of the TX area HSB fanatics will have the best idea as to how many to add. I would estimate you could go with 100 HSB if you are not planning on feeding, more if you are going to feed. If this is your first pond I would stay conservative and hold it to 200 HSB or less this year. If you like them as much as most of us, you will be adding more every year or so.