Hi Buster,

The MST Hub is a DP++ device, so you only need to use passive adapters. The only exception here is when you use a resolution above 1200p then you will require an active adapter.
In regards to your second question, can you tell us what graphics card you have? It really depends on the supported chipset.
For example, for Nvidia GeForce graphics cards the maximum supported displays are 4, it does not matter what combination you make. 
On Radeon graphics you can run a maximum of 6 displays.
When I mention the number of displays supported this is on MST or Multi Stream Transport, each display running independent with their own OS window.
You can run also in SST Single Stream Transport (depending on the graphics chip) which it will mean that you can run lets say 3 screens with a single resolution expanded. So if you have support for 6 MST on a graphics card, you can run 6×3=18 displays. Note that this can be limited due to driver support or graphics chipset.
Please let me know your graphics card make and model and hopefully I can help you.
Regards