I'd be reluctant to add anything under a foot, a 6" bichir will be a snack in 6 months time when the aro will be pushing 14"+. Even bigger bichirs like endli would have little defence against an aggressive jardini though, there's a good chance the jar won't get on with any bichir, regardless of size.
I'd go with a bigger Bichir. Have a spare tank on hand, just in case the Jar starts attacking the Bichirs. I would definitely provide a good amount of hiding spots. Jars are known for killing their tankmates. It's a crap shoot if you want a Jar that will tolerate other fish around it. Good Luck!