whonneb,
I'd program in all the frequencies for the site. A PSR-500 has the ability to do CC only trunking (but if you're going to the trouble to change 2 frequencies, you might as well change 10 real quick). The reason I say this is because you never know when any frequency can be chosen as the CC, and you'd be cursing yourself for not changing a few more frequencies. In most cases you'll only need to change the digits before the decimal (15 MHz less).
I'm not sure what you mean when you ask "Also does anyone know what is the best mobile scanner for the gre psr-500?" I think the confusion for me comes in the vocabulary. The PSR-500 (I'm assuming you have one) is a handheld scanner. Mobiles are the ones made to fit in vehicles (DIN sized), and the mobile scanner equivalent of the PSR-500 is the PSR-600 (or Pro-197). So I'm unsure of what your question refers to... unless you're asking about a mobile antenna for your 500 (but I didn't see the word antenna either).
Let us know and we'd be glad to answer your questions here...