Can I use a radio firmware for GB (2.3.6) for my SGS1 (GT-I9000) if I have ICS installed? I still have the original cellular radio firmware my phone came with (at least, I think I do. The baseband version is I9000BVJJPG).  However, I've changed ROMs many times, and am currently using the latest AOKP Milestone release.

Is it safe to install the latest radio image from http://samfirmware.com (PDA: I9000JIJVG, CSC: I9000CELJVG), if it is meant for 2.3.6, and I have ICS?
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.