@Nico33 

Imho in most cases a sub mini will fit well with Sonos Beam. Just if the room is very large a sub GenX would be the better option. But then a Sonos Arc probably also would be the better choice. In the end it also depends on your personal likes of sound. If you like a very powerful bass, you might also go with a big sub. 
For placement there is no general best option. Each room is different and you have to find the best place by testing. Put the sub at your usual listening position and walk around the room to check each position where the sub could be placed. That one where bass/ sound is the best for your ears will be the place to put the sub. 
Sub Mini imo also profits from placing at a corner of the room than putting it somewhere to the open room or just at the flat wall. 

Low frequency sounds are less “directional” than higher frequencies. The nature of sound and sound waves means that repositioning the Sub/Sub Mini just a small distance can make quite a difference to the sound you hear. Experimentation in your room is the only way to hear - and decide - what you like. 

I can not have the arc for space reason. So only the beam can fit so then maybe the sub mini is enough.

thanks all for explanations

I use a Beam2 with Sub Mini myself and I‘m pretty satisfied with it. Did a „downgrade“ from Sub Gen2 because of dimensions and design and for me it’s not such a huge difference. 
But just my personal experience.