1. I was wondering if i have a lot of old sites in the same theme which i dont use it anymore but have some history (backlinks i mean) should i do redirect 301 to my original site. Assuming i have 30 - 40 domains, will this cause any trouble such as penalize? (keep in mind all this in the same theme & same industry. <snip>
2. Would it be better if i create a unique content site for each of thise 20 - 30 domains and do linking to my original site?
[edited by: Receptional_Andy at 3:02 pm (utc) on Nov. 21, 2009] [edit reason] No specific keywords or industries, please, see charter [/edit]
You're unlikely to be "penalised" for redirecting inactive domains to a central location. The biggest risk is that doing this fails to consolidate the value.
It seems like it would be a great deal of work to create a site of value in each of 30 or so cases, but if you think you could crate sites that Google would rank independently, then that would be the way to acquire the biggest share of your target audience. It's difficult to see how you could create so many sites of value though.
I have clients with over 1,000 domains that are 301 redirected to a core domain, with no problems. As long as the trust profile is strong, it's fine. Just don't continue to promote the legacy domains.
Not all the back-link value will transfer - it seems to be around 80% to 85%, at a rough guess. And in some cases it may take a long time to see the old link juice kick in for the new domain, especially if there is something about the redirects that looks strange to Google's trust routines.