Although your question is too long and quite difficult to understand, but after making 2 review that you are wanting a sub category for specific niche. In my opinion you can do so if it relavent to that 2nd keyword. If I misunderstood sorry for that. Thanks.
http://forums.digitalpoint.com/showthread.php?t=1447662 it is all about the domain factor and much more about built in directories offf of the main index to build traffic.... im trying to stay unique as possible ... do give expert advice lol greenies :D
