Hiya, welcome to Filmwasters. I'm glad people are talking about us in a complimentary manner.
I can help with this question (for once). I've used the GSN Electros for years and you're absolutely corrent when you suggest changing the ISO rating. It's the standard way to compensate for filters with the Yashicas. I've used red/orange/yellow filters and a polariser with my two Yashicas and in every case you simply adjust the ISO....and hey presto.
I don't think I've used IR film with my GSN, but I have used IR film in other cameras and any dark red filter seems to produce some sort of effect. These two for instance were taken with an LC-A and Kodak IR film. I used red plastic from a stage lighting gel as the filter. Held in front of the lens. I guess the more scientific you are about it the more control you'll have, but I usually just get stuck in and improvise.
The main problem I've had with IR film is not actually shooting with it a getting IR effects, it's been finding a lab to process it (assuming you're not doing so yourself....which is a bit presumptuous of me). Not all of them will do it due to issues with the safe lights they use in their darkrooms.