This site has lots of problems.

I don't know what terms you are trying to rank for... but... if this was my site I would...

1. remove the links page from the navigation and would probably delete it from the site

2. decide what is important and focus my homepage title tag on that (it is currently way too long, suffers from keyword stuffing, repetition, does not elicit clicks)

3. rewrite the homepage, eliminating the bold and color text from the paragraph copy

4. fix homepage canonical problems ("home" goes to index.htm)

5. blog is on a subdomain but the link in main navigation goes to index.php in a folder

6. move the blog into a folder

7. get rid of the tags in the blog

8. create alt attributes that support your keyword targets

9. develop a persistent navigation that directs visitors to your essential pages and employ that navigation on the main site and the blog

10. develop article content for each of your business services

All of that is from a quick look, there is probably a lot more that a detailed study would find and which I am unable to offer.

If you are getting business from this website I think that it would be worth the investment to have a professional SEO spend the time needed to learn about your business, determine the keyword targets that will attract business, help you get it optimized, and develop a content strategy.  (I am not looking for a job, I only work on my own sites.)

The reason that I would hire someone is that it will get the job done right immediately and eliminate the loss of revenue that will occur while you are learning.  Or, you can find an SEO who is willing to consult and coach you while you do the work yourself.

Good luck.

If the number of pages on this website increases to 10,000 can that decrease its domain authority or affect it in any way?

Are you adding gold or crap?    Gold?    Crap?

Find out how much traffic you would be walking away from if you block them from the index.   My category pages bring in huge traffic - more than most websites receive.  I sure would not block them.  Also those pages attract a lot of links, likes, etc.  If I block them from the SERPs those benefits would be gone.

My advice is to understand the website better..  Then make a decision based upon data.

I have no opinion on this.  It might work, it might not.  I would not do this on any of my sites.

I think you are starting with a small amount of seed content and spreading it very thin through many pages.  I can't imagine how this would produce a good experience for users.

I think that they can be harmful to your site - especially if you use keyword anchor text.

If these are the only types of links that you have, I think that your site will be hit by Penguin.