Advice about adsense site

First of all as you said you are a newbie, Do you have an approved adsense account. If you choose to do adsense on you own domain/s then google wants that domain to be at least 6 months aged, good content (obviously).

If you don't have one I would suggest create a blog at blogger or hubpages. Write posts or create hubs about 15-20 (spread them, couple of posts a day). You want to look like a regular blogger.
Also try to optimize those blogs and get some traffic as you would do on your own site.
Your blog should not look like it's made for adsense. Once you have some posts on your blogs and have some traffic coming in then there is an option in both hubpages and blogger that says monetize your blog. Go apply for that after about 20-25 days and you should be ok.

Once you have approved account then you can use the same account for as many domains and sites as you want.

1. 1900 and 8100 are more than enough but make sure that they are not too competitive to rank.

2. To check if the niche is good for adsens or not search for the same keyword in google and see if there are adwords ads and if you see ads on google than that's a green signal there to start.

3. How much money you will make is something no one can't tell you because that depends on the niche. For example if you content is all about let's say "weight loss" then people bid about 1-2 for each click on those keywords then you can make good money from those but if people bid let's say $0.50 on your targeted keywords then right there you reduced your profits by 50% as compared to weight loss or if the bids are like $4 you increased your profits by 100%. I hope this helps. Let me know if you have any questions.

I had my adsense setup week after I setup my website.

1. Are you thinking about making micro site? For micro niche sites, I usually find a keyword that has 2000+ US searches with $1+ CPC. I try to find a keyword that also has exact match domain name. Try using adwords keyword tool and godaddy at same time.

2. I use any kinds of niche. Anything I can think of. For example, I was smoking outside my house today and someone was installing solar panel on their roof and that became one of keyword. I would go in details, but since people sell WSO regarding this topic, I won't.

3. Well, let's say you have 100UV and converting ratio is around 1%. You will have 1 click a day and a click can be from low as $0.05 and sometimes over $1. There is also strategy for this but I can't talk about it since people are selling WSO about this tip. So 1000UV with 1% CTR can be $0.50 lowest. 10,000UV can be $5.00 lowest, etc.

You can PM me anytime for details.