QoS and management of traffic on subnet - slightly OT

I am having trouble managing traffic on my network. The problem, in short, is that one guest can, in some circumstances, hog our very limited (40Mb/s) bandwidth. For example one guest downloading a massive game on an Xbox can take up most of the available 40Mb/s depriving another guest from watching a video on TV.

I have been using a Sophos SG135 UTM as the network management device as this has the facility to manage network traffic using its’ QoS capability. The process on the UTM QoS is to use a Traffic Selector tool to select the source, destination and service and then apply a filter using a Bandwidth Pool to restrict traffic.

I have a couple of really dumb questions:
Using the configuration tool for the Traffic Selector if I select source as ‘any’, service as ‘any’ and destination as our guest subnet and then apply a filter of 4Mb/s this seems to work. I had thought the source should be a guest on the guest network. In other words the other “direction” with source being the guest user and destination the internet. Why am I wrong?

My second question is that with the filter in place and working I can see from the traffic log graphs that the Guest subnet is indeed restricted to 4Mb but what happens if a second or third guest needs to use 4Mb/s bandwidth. How do I configure it so each individual using the guest network can get a share of bandwidth?

I appreciate this is well OT but I do need to understand how these kinds of filters work. If anybody has time and the interest I would appreciate help and advice please.