Wow, I'm planning on filling the libraries with several playthroughs since I'll definitely have more than one file/to get all the S supports. If you can't decide how you want to pair them up from supports alone, I'd say rule which are most useful on the battlefield together- so they'll actually get used. And I'd recommend pairing up the units who have a limited pool (Chrom and Sumia) first, the going on to your MVPs. For your Avatar, might I ask if you like Gaia's supports? I dunno if they're the same as the fan-translations I read, but if they're anywhere near them, you're striking gold. Also Libera's S support is pretty darn funny too.