See this thread for MAK key usage: Re: Windows 7 VDA license, three days for activation message
As far as activating using your KMS server, yes, you need to get to that 25 limit somehow, and I ended up doing it the way you're suggesting - syspreping a bunch of machines multiple times to get to the 25 required.
Can't see any reason why you couldn't add your WIN 7 KMS key to the 2008 server, and if you want to activate office using KMS then you'll need to add the Host License Pack before you can add the Office KMS key.
Geoff