Can I use a radio firmware for GB (2.3.6) for my SGS1 (GT-I9000) if I have ICS installed?
You do not want to do that. 

From my understanding, flashing an older radio than the one you currently have installed is a sure fire way to brick your phone. 

Besides, the latest radios are always better. And so long as you are sticking with ICS, it is a very good idea to use the latest radio. 

Like ce4 said, flash the radio recommended by whoever is making your ROM- 
And NEVER flash the same radio twice.