You can certainly intermix various connections (cameras, focuser control, mount control, etc.) on a single USB channel but you are probably better off splitting things up a little bit. I think it helps to sit down and think about the volume of data each connection is going to generate and any power requirements a connected device might require.
- Mount control is usually a serial protocol that requires very little USB bandwidth and very little power (it's the USB-RS232 serial adapter that uses a little bit of power). Even continuous autoguider move commands don't keep the USB channel busy at all.
- Focuser control is similar and only occasionally uses any bandwidth at all. The focuser control interface is often powered from the USB channel.
- Camera control, usually also serial connections and use very little bandwidth. Like the mount there will usually also be a USB-RS232 adapter drawing a little bit of power.
- Autoguider cameras, these use more bandwidth and those with higher frame rates and higher resolution sensors being the tougher combination. These cameras often also draw a substantial amount of power from the USB channel. The combination planetary imagers/autoguiders tend to be the higher resolution models with lower sensitivity sensors and probably put the most
- Video cameras (useful for finders, or in some cases as the primary camera), the higher frame rate (25-30fps) use bandwidth continuously but generally these are not high resolution cameras so the combination isn't too bad. Video cameras are externally powered, but the USB frame grabber used to convert video to USB does draw a little power from the USB channel.
- Imaging cameras, these put real pressure on the bandwidth available but only for very short periods of time (the image download). Power is usually not an issue as they are usually externally powered. Because of the intermittent download it can appear that everything works well on a single channel but only every once and a while you get a "freeze" condition. This can often be caused by the occasional overlap of all the device on a USB channel transmitting together.
At the minimum I would consider using two USB 2 channels to separate the two biggest bandwidth users. One with the autoguiding camera and some of the low speed connections (focuser, etc.). The other with the primary imager or video and the mount control. This will also help when you need to troubleshoot problems (being able to swap things between the two channels). You should watch the power requirements if all the devices are USB powered. This is where a powered USB hub can be very useful. If the attached devices are externally powered (or draw minimal power) you can sometimes get away with using an un powered hub. The USB spec says that the maximum power supplied to a channel is 500ma. Some channels (particularly on laptops) can reduce this amount or completely shutdown the power to a channel to conserve battery life. Most of the time this won't affect operations (as the power should be available when the channel is active) but if you have intermittent problems with devices like cameras generating a lot of image noise it would be something to consider. USB channels or ports that are siamesed (mounted together) on a laptop can also sometimes share the 500ma power output. Putting the two USB connections on two separated connectors can help in this situation.
Another suggestion that doesn't affect bandwidth and such but will help when troubleshooting is labeling connections and keeping cables and devices on the same ports (unless you are troubleshooting a problem). Intermittent USB problems can be really difficult to find and keeping the connections consistent will really help the process and your sanity.