I was improving the pagination on my site last weekend. These are answers from my perspective as an e-commerce site owner.
- "- Since my USERS always stay in the same page (they never should see pagination pages, those are for crawlers only, since we're using Ajax), I think I can consider our "first page" as a "View-all" search result page, right ?"
I suppose that's sort-of true for your USERS, but not really for crawlers (including google bot). They will still see a paginated list of items. A "view-all" page needs to show ALL the items on a single page.
- "- If I'm right, this means I should NOT use rel=next and rel=previous, instad use the tip in the first article, and use rel=canonical in every single one of the pagination pages to hint Google to always display my first page (which is not only the first page, but also the see all page, because there's ajax pagination working). Am I right?"
The problem with using rel="canonical" is that you don't really have a "view-all items page" but still a paginated list of items. Each page will likely have DIFFERENT items on every page. Since the content is probably not the same on every page, rel="canonical" is probably not the best choice. I think using rel="next" and rel="prev" is the better option for a paginated list of items and gives a more appropriate hint to google about the structure of your site.
This is a quote directly from the google webmaster article you sited above:
"It’s not uncommon for webmasters to incorrectly use rel=”canonical” from component pages to the first page of their series (e.g. page-2.html with rel=”canonical” to page-1.html). We recommend against this implementation because the component pages don’t actually contain duplicate content. Using rel=”next” and rel=”prev” is far more appropriate."
Of course, if you build a true "view all items" page, then just do what the article suggests 
- "- Should I add meta noindex to the pagination pages ? (I wont' be usingle nofollow since the point is exactly the oposite, that crawlers follow the links in the pagination). "
I think rel="next" and rel="prev" should be a big enough hint, but if your 2nd page and onward is really ugly and you don't want anyone to ever see it, and you're sure the ajax pagination will always work, then yeah, noindex it. Otherwise, let google decide which page shows up in search results (helped by next/prev hinting). Who knows, there might be something relevant to a USERS google search on page 6, why prevent the user from finding it?
4)"- Should I worry about CSS and layout for these pagination pages ? right now they only print the HTML "snippet with the links to the main search page.
I would make the paginated content look/feel the same as the rest of the site. Why? Again, maybe there is something relevant to somebody's search on page 6? Make it look like the rest of your site when the user lands there.
Cheers-
