When I had Gray Fox, I added CSPs, CSP name, small picture, map icons, and his in-game portraits.
I used AS1nk's Custom Red CSS for PAL users. The only edited text I have is that it says "Mewtwo" instead of "Lucario." I strongly know that it doesn't cause my Wii to freeze. I inserted my stuff into it, yet it still freezes...
Hmm, you said ASF1nk's Custom CSS; do you have Disable Custom Stages code by any chance? I experienced freezing, albeit at the SSS, when I first used this code without Disable Custom Stages. If you don't have it on, try adding it and see if it works. If you do have the code, are you sure all of these files are the same dimensions and formats as the ones they're replacing? If you say yes to that too, then I'm sorry but I don't think I can help you any further.