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. 

I recently purchased the Arc Ultra and debated between pairing it with a Sub Gen 3 or Gen 4 or the Sub Mini. I have a decent-sized living area (22’ x 20), but it’s not an empty space. The amount of furniture you have in the room makes a difference. I went with the Sub Mini (upon Peter Pee’s suggestion on YouTube), and used his suggested EQ settings, which helped a lot. I find that when paired with the new Arc Ultra (and two Era 100s), the Sub Mini provides more than enough bass for both movies and music. The bass is very deep. I don’t need to have a level of bass that shakes my entire apartment and I also want to be considerate to my neighbors. I’ve lived next to neighbors who had very powerful subwoofers and that be a nightmare. I didn’t test the Sub Gen 3 or 4, but again, I find the Sub Mini provides more than enough bass, even with it set to -1, and the bass on my Arc Ultra turned down to -6. It’s still enough.