SEO question, multisite sites linked to main site


How bad or good for the SEO purposes if...

I will create main website with special topic and...

On the same wordpress instal I will make different blogs related to main site and put links to main website.

I want each blog to have its own domain with domain mapping plugin. Also I ll do my best to rank each blog with little bit different keyword than main website.

Any idea what google will see... and how it is good for SEO purposes? Will I be penalize?

Thank you