Thank you all!
Those mikrotik's can run other linux distros? The folk that told be about that said that they coulde use an ARM or x86 Architecture, and could run windows or ubuntu. But as far as I can see, those are dedicated for routers, right?
Which mini-pc's like RPi support PCI-E? Not much, I would say? Do you know any?
For the cheap-tablet: that could work, but with small pc's we have has many USB's as we need, without more HUBs, and they are also smaller. Here, size matters :/
Best,