Hi John,
Yes, you'll need both; but they are small compared to something like Visual Studio. Speaking of which, you'll need VS Standard or better for the current implementation. If you use VS 2008, the DSL tools are already covered in that install (the reason you don't see them in zip folder for that package). The PLiX is essential for all the generated mappings (I don't know if you could run the tool at all without it - but you'd lose all capabilities to generate DDL, XML, etc...).
I liked some of the aspects of InfoModeler too. In all, ORM2 is better; and the nORMa tool has come along way. Do keep in mind that it's still CTP, so your feedback, added to others here, will help the team. You may not know that the admin of this site, Ken Evens, is a co-author of the definitive manual for ORM in Visio for Enterprise Architects; a source for your questions about that tool.
If you have VS on your taxed system, the tools install isn't likely to be the straw.... If you do need to clean things up, consider taking out some parts of VS that you know you won't need. Lot's of people load up stuff they'll never use as part of the default install. I'd bet you would save much more than the cost of adding DSL and PLiX.
BRN..