Yes, the Arc can be used with the Sub Mini.

Yes, see this link…

 https://support.sonos.com/en-us/article/sonos-products-that-support-sub-mini

Hello. 
Yes, I have mini sub+arc+2 one SL installed and it is perfect for me in a 25-meter room. The sound is brutal being a mini sub,

I can't turn it up more than 45% of the volume so as not to disturb my neighbors. At first I was doubtful whether to buy Sonos sub 3 gen or sub mini and I was right to buy the mini.
If I buy the Sonos sub 3 gen the neighbors will throw me out of my house.