I can assure you that most networks, including MellowAds require an Alexa rank of over 1.5 million, which takes 50,000 or so impressions a month.
A-Ads is one of the best that have low requirements. If wanting to mix advertisments with BTC gained off page visits rather than only originating from clicks then the two mentioned you could mix and calculate what affiliate programs can boost income potential.
Overall, looking at who's advertising onto banners will lead you to going direct until built up enough to get through the application process.
they do NOT accept free blogs, and that is the problem.
i am not worried about alexa rank if it was only that, i would have made it. and although i am new to this, my blog already has 42K visits and it is about 2 months old.
and what you said about a-ads is not going to work. the affiliate program only works if they click and sign up and pay to advertise with their network. and people usually don't do that!