Can you tell me if the fuse would be on the positive line?
Yes, you place the fuses on the positive line out of the supply.
Also I don't think I understand the bit about separating the power between the hub powered node and the injection powered node. Why is this important and how would I achieve this if I am using one of the T connectors?
In many cases it depends on why and how you are power injecting. If you are injecting power simply to stop the end of the string from dimming and you are using the same power supply on all of the injection points, then separating or cutting the power line between injection points is not necessary. If you are using different supplies and / or injecting due to current limitations, then it becomes necessary to separate the different sections from eachother.
The primary problem is if you don't and one of the power supplies shuts down for some reason (and it does happen), then the entire load will be placed on the other one. If the reason that you are injecting is due to a high current load, then you will be pushing the other power supply and / or its cable beyond its limits which can cause a fire or failure. With the hub, that may result in a blown fuse and they ae not all that easy to replace, especially if the hub is already connected into an active display.
Another issue is based on the reason why it is strongly suggested that only single rail supplies be used with the SS hubs. If there is a difference in the output voltage between the two supplies, they may fight each other to get the voltage to were they are set to operate and PC supplies are somewhat sensitive to this and are tightly regulated. Low cost power supplies tend not to be such and may cause the PC supply on the hub to work harder than necessary. It is also harder to control how much of the load is handled by each supply and the one with the higher voltage will also take most of the load.
Separating the sections eliminates all of the above problems. Each supply only powers the section of the string that it is connected to so you have com[plete control over how much of the load it is powering, the supplies never see eachother so there is no fighting if they are not operating at exactly the same voltage, and if one of the supplies cuts out, the other supply is not affected by excess load from the rest of the string.
I don't know what you mean by T connector but if you are referring to a line tap, then just cut the power line on the side of the tap that the hub is connected to.