The answer to #1 is yes you can remove these as the concrete slab will and can do the same function that is support the posts.
You should plan on re-enforcing the concrete below where the post will be setting, completely down to the footings.
There are methods of attaching posts to concrete slabs. Some have been posted here on the forum in other threads. You'll need to do a search for post to foundation connections.
Also, there are post to concrete connectors sold by Simpson strong tie company. There is a link here somewhere to their site where you can find all types of connectors for this type of use.
As to the weight factor if they can't handle it they might have to be custom made by a local welder in your area.
As to the grades of lumber, I'm sure you can understand that you or your sawyer can mill a 2x6 from your own stock with a 3 or 4 inch knot in it. But it wouldn't be strong enough for just about anything. This is why grades rules were adopted. You could get a rule book from the Western Wood Producers Association and read up on the rules for grade 2 and once you understand them you can then select and mill good trees into proper strength timbers.
If you aren't going to personally mill the timbers then talk to your sawyer and see if he understand these rules. If he doesn't then either you have to educate him or find another sawyer who does understand them, and have him mill your timbers.
As to whether or not you can substitute Douglas fir for oak, you might have to have a structural engineer review this. You'll need to find one from your area who is familiar with timber frame construction. You can go to the TFG home page and select the resource guide and do a search for engineers and see if there is one in your area.
Good luck with your project.
Jim Rogers
