Based on what I've read about the other providers that use the same Mediaroom technology that MaxTV uses, the providers implement QoS (Quality of Service) on their gateways so the Ethernet ports that connect STBs get priority over devices on the other ports. When you mix both on the same port, the gateway isn't able to ensure the STBs get priority. This is important because Mediaroom uses real-time UDP protocols that depend on a reliable connection.
I've heard of a lot of people using switches and only a few problems, sometimes related specifically to Apple Mac computers. Any burst of data could disrupt the TV stream, such as loading up a buffer when streaming a local video over the network to another device. For this reason I wouldn't recommend putting the PVR on a switch, since it handles a stream for each program you are recording plus each recording you are playing back, plus potentially each live stream since each time you press pause on a remote STB, the program gets buffered to the PVR, resulting in 2 streams going through the PVR.
There are many ways to get 7 boxes on a single gateway. In the early days many providers used existing cable-TV coax that plugged into a special port on the gateway. Now, MaxTV supports up to 3 wireless STBs in addition to the wired boxes. Before wireless STBs were available providers such as Telus have supplied switches to provide additional ports, but they only connect STBs to those switches, so the gateway can maintain QoS for the TV streams.