Hi,
You should be able to install Visual Web Developer Express along side VS 2008 and VS 2010, and it has everything needed for mojoPortal development. I have VS 2008, VS 2010, and Visual Developer Express Phone version all installed on my machine.
What features do you need that are not in the Express version?
You won't be able to use VS 2008 with the latest mojoPortal code, so maybe you can use Visual Web Developer Express when working with mojoPortal and VS 2008 when working on other projects.
As far as I know there are not much differences between the Express version and VS 2010 Pro, other than no third party tools can plugin like Coderush or other add ons can't work with Express, but other than that it is about the same.
Hope it helps,
Joe