@moriond
I have read the different threads and follow the links, and with your help and the intervention of the various participants, I think I actually understood! Thanks again for that. I had a little trouble getting by with the (past and current) different designs of Side Kick and the different straps but that's ok. A lot of interesting information in these answers and suggestions.
This is great news that it is possible to use the shoulder strap for the Side Kick! And, I will test both the included Shoulder Strap and the Gatekeeper Waist Strap that i will order.
I spent a lot of time looking at the accessories and different pouches. I chose some of them, then after your message I reconsidered the different options and added them to my order (by the way I had already chosen to order the strap keepers!

).
It is not so simple to visualize the different sizes, especially for a bag that is not yet in his possession.
But I think the range of choices I made will work well. And I made sure that the pouches I chose could be complementary to the Muji pouches I already have and use.
I was also inspired by
My Side Kick thread: In detail to choose some accessories.
The only thing that made me hesitate is about the Minimalist Wallet. I love the principle, which I have been looking for some time.
But I still use coins (and I think the Clear Organizer Wallet is too big.) My current wallet is the
Lemur Wallet, and I do not want something bigger.
Then, I'm still thinking about it.