It depends on the size of your room and the level you listen to audio . If you have a smaller room and need to keep the volume low, the Sub Mini would work fine with the Arc and Arc Ultra. But if you tend to have the volume a little loud, the full Sub may be the best solution for you.

Look here is the FAQ:

https://www.sonos.com/en-us/shop/sub-mini

Sonos’ FAQ’s confirm the Mini is compatible. But the full-size Sub is likely a better match with the Ultra. 

Went ahead bought the mini,and for the money( $300.00 total with 30% off) it suits our needs perfectly!

Does it rattle the floor? No,but we can tell the different when watching movies.

What size is your room? I’m also considering going for the mini with the ultra. Our living room is about 24 square meters.