Hi Karsten,
First off, welcome to Rockstor community. Look around and you’ll find some useful information and feel free to share your knowledge and ask questions.
Rockstor will boot from USB. But I see that this mobo doesn’t have usb 3.0. So things will be slow. Some optimizations are coming in 3.8-3 update which will help. If performance becomes a major concern, you might want to consider something faster via PCIe, but I digress. If you do want to go this route, let me know.
Regarding your expansion plans, your assumptions are correct. But please be aware of the current status of RAID5/6 maturity. @Tom summarized it very well here: Question about RAID5/6 stability