As you have probably already figured out,
Shutter Stock,
iStock,
Dreamstime and
Stockxpert are the hardest to get accepted to. I would wait to do those until you get a good idea of what will sell as stock. I know Shutter Stock makes you wait a month to resubmit if you are not accepted the first time around, I don't remember if any of the others make you wait between submissions. Another good one, after Fotolia, to get your feet wet with is
Big Stock Photo, They are pretty liberal with what they accept but you will get occasional sales there.
As far as earnings go, it's like anything else, you can only get out of it what you put into it. $300 month after 4 months isn't impossible but it is a pretty lofty goal.
Hope this helps.
Matt
PS. Feel free to use any of my referral links(I don't have any yet

)