Multiple installers attached to a single GPO?

It most certainly can be done, but you are not guaranteed of the order they're applied. You can set prerequisites which can help handle the dependency tree, but otherwise it's non-obvious what order they get thrown on machines.

Yes, it can be done, and personally I always do this where possible. One thing that I've remembered from Microsoft courses about Active Directory when Windows 2000 was first launched was to keep the number of policies applied to an object in AD to an absolute minimum, as lots of policies could cause boot/logon delays. As a side note, if your policies only offer computer settings, disable the user settings, and vice versa.

When you use GPO filtering or delegation permissions to target your software deployment to different classes of computer/user, this is done per policy, so there will likely be instances where you can't avoid splitting up your software deployment into multiple policies.

As per sysadmin1138's answer above, if you need to specifically install in a certain order, forget it, unless you have a good set of MSI editing tools to create dependencies within the MSI installers. I've used InstallShield admin studio to do exactly this when Visual Studio .net 2003 was deployed to about 400 systems, this allowed me to ensure the prerequisites were installed before the main app was installed, without this the installation would fail completely.

You'll need to find a freeware app called "Assigned Software Sequence Manager" from a company called Sywan ICT. The order that the MSI's are installed within a single GPO is dependent on some date/time-based setting that the usual AD tools don't give you access to. Sywan's app lets you order them as you want.