I don't know about that cause a year ago my fellow legendary countryman applying for merit source but sad to say didn't accepted. And I don't know also about this if low ranks applying for Merit Source got accepted. Anyways that's not the problems here we can Post in global section and get merit.
As far as I know, the lowest rank that got his merit source application accepted was Full Member, so it's not like you have to be Legendary member with thousands of merits earned to get accepted. But it makes sense that merit sources are usually higher ranks (Sr Member and up) as it mean you already shared quite a lot of merit and have some experience with it, and you had time to build forum reputation which is also important I think.
There is no rank criteria to become merit source and the merit sources information isn't publicaly available so we also can't say that newbies can't be a merit source but the criteria from theymos is very clear collect 10 worthy post which deserve merits and you should be a recognised member in the community.
Those two things contradict really, being a newbie and a recognized member of the community. So it's safe to assume that no newbie has chance of becoming a merit source.