You can mix downloads and disks.
In my experience, that isn't always the case. I had to reinstall my game recently. The base game and earlier EPs and SPs were all on disc, while the later ones were downloads. I was able to download the two EPs just fine, but when I tried to download the two SPs, Origin gave me the message that they were not compatible with my game. Why this should be I have no idea, as I had them installed before. I tried Origin Help and they weren't any help at all - they just kept insisting that the downloads were compatible with the base game, which is a little ridiculous when Origin is telling me they aren't.
So I am having to do without two of the SPs that I paid for. And I will never buy a download again, because I don't want to go through this again. So I think people should be cautious about mixing the disc and download versions.