Redirecting them to the primary domain will only help in direct navigation. Mirroring them will result in duplicate content.
What is the worst that could happen if one of your mini sites outranks your main site? Having 2 of your sites on the front page of the SERPS is better than having 1 of your sites and 1 more of a competitor's site.
In my case, I have widgetworks and rank well for "widget software". I am working on a mini site for "gadget software" and putting it on gadgetsoftware.com. I never have ranked at all for the gadget keyword while that is my competitor's leading keyword. I figure I can rank high for my primary keyword and this mini site can rank high for my competitor's primary keyword.