Im trying to use a 64 gb Patriot xcsd in my S2 (which officially supports 32gb, I know) but I found a few people saying all you have to do is format to fat32. I did this but it still doesn't read, but people are recommending to do this with san disk cards. Is there a difference between the cards?
Not usually, but the S2 has a tendency to be picky when it comes to large cards. Most people have luck with either Samsung or Sandisk in the GS2, but then again, YMMV. Try AParted and see if it helps: https://play.google.com/store/apps/details?id=com.sylkat.AParted - if AParted is unable to see the SD card, fire up your computer and try creating a 16GB partition on the card (fat32 of course), then try AParted again (it should see the unformatted volume, if the card is truly compatible with the S2).
If AParted didn't work, try flashing back to Android 2.3 (stock or CM7) and formatting the card on the device itself (to my knowledge, external SD formatting can't be done on Android 4.0 or above), then update your rom to whatever you were using before you downgraded if it succeeds.
If none of that works, try a different card. If another card doesn't work, it may be time to replace that phone if you want fat storage.