I'm new in this area, so a few small questions. Lets say a site generates large quantity of traffic (around 20,000 uniques a day), but it's not targeted towards anything specific and it's not coming through SE. The users will stay on the page and will see ads (the page gives them a service they need). Is this considered good for using affiliation links? Anyone knows if it suppose to convert well? Are there people/companies who specialize in finding and setting up the right affiliate programs for this kind of a crowd?
Depends on what the crowd is and how they're getting there! It could be useful but less so if it's not authentic traffic. It really depends on who and how but ultimately; you'll only master your mystery traffic via trial and error. Best of luck.
Get those non-targeted visitors to a squeeze page and get them to opt-into your list. Send them a series of emails convincing them about a general niche (e.g. making money online) product and you'll find 5-10% of them will actually buy it if you do your job well in convincing them. Felu
If the visitors stay at your pages, try Adsense or something similar for a few days. If you'll get clicks, let's say CTR > 3%, be sure that they are interested and you can put some affiliate links