Wow you really have a good selection of older MDT pumps. If my memory serves, you need to have a certain firmware on the 523 to make the loop work, but the 522 works on all firmware versions.
Currently, only the following Medtronic MiniMed models allow us to remotely set temporary basal rate commands, which is required to do OpenAPS:
512/712 (all firmware)
515/715 (all firmware)
522/722 (all firmware)
523/723 (with firmware 2.4A or lower)
554/754 (European Veo, with firmware 2.6A or lower; OR Canadian Veo with firmware 2.7A or lower)
On your question about Xdrip vs Nightscout. Nightscout is just a way to get the data out of your system in a usable way. I don’t think it makes any predictions. We set up a nightscout site, but my son wasn’t very interested in it. I think you have to compile your own software solution (reduced liability) to do the predictions. Here is a link to a thread talking about that.