Yes, you can connect monitors that are not DisplayPort 1.2 monitors to the outputs of a DisplayPort 1.2 MST Hub. You can even connect adapters (DVI, VGA, HDMI, etc) to the outputs of a MST Hub. The following is from the page of a MST Hub manufacturer (link):
The Club 3D MST hub is a hub device that receives a DisplayPort™ 1.2 MST signal from the source device and splits up and routes the video streams independently to each display device. Using this type of configuration also allows the use of non DisplayPort™ 1.2 monitors. Non DisplayPort™ outputs, such as VGA, DVI or HDMI®, the Club 3D MST hub is DP++ and can actively convert the DisplayPort™ signal to the other types of display interface signals.
One other possible issue is the GPU and drivers on your machine which may or may not support multiple external displays. For example the Surface Pro 2 with a Haswell chipset had driver issues which prevented the support of multiple external displays at launch, despite supporting DisplayPort 1.2. But these particular issues have since been fixed with a driver update in March 2014.