Choosing a frequency for the SMs/APs

We are running a 900MHz network and there is also another companies 900MHz network within about 4.5 miles from our tower. They are running their 6 AP cluster on frequencies 906,915,and 923. We run our 6 AP cluster on frequencies 907,914, and 923.
I have 2 questions:
1) Are we setting up our cluster correctly?
2) We we set up a SM should we set it up to connect to all 3 frequencies or just 1 frequency, whichever one gets the strongest signal?