Let says I have an ecommerce about dog food. My url would be http://www.dogpalace.com Now I have grab the domain http://www.dogfood.info . Would it help to make a single page website on this info domain with a few links back to to my ecommerce site ? I could also promote the info domain to help get him better serp ?
yes, you are right, if you use same keywords, chances are more to be good in SE back links will help you to increase page rank. Search engine works with pages, not with website
if the are 2 independant sites with same or similar content, then a link from the info site will help your .com site. but you have to do some promotion and link building for the info site so that it can pass some PR to your other site and google can find the backlink to your other site.
It can, but you have to be careful about it. Obviously the dogpalace.com site would be commercial in nature, while the dogfood.info site would be informational in nature. One thing you may want to do (and this has far more to do with Web copywriting than SEO) is to drive people who read the informational site to your commercial site by writing your Web copy to promote the commercial site while masking the copy as an editorial. There are risks involved, but if you pull it off, well, I'm sure you can figure it out. If you need to learn how to do this, I strongly suggest you go to your local public library (or go online) and get a copy of "Web Copy That Sells" by Maria Veloso.