Well, 'meg' is like 'and the other one is', so 'Ez lány, ez meg fiú.' could be translated as 'This is a girl, and the other one is a boy.' But it's hard to translate 'meg', we use it for a lot of things. By the way, we NEVER (at least the literate part of people) use 'this' to refer to a human being, it is rude. So, try not saying 'this' to another person.
If you use 'ő' you are always on the safe side. However, when showing a drawing or a painted picture, even the highest educated folks would use 'ez'. On the other hand, in colloquial, meant-to-be-funny conversation the use of "ő" is spreading for lifeless objects and animals as well: 'Ő a szomszéd macskája." "She is the neighbour's cat." "Melyik a legjobb boruk?" (What is your best wine?) "Ő az." (This one.)
As the other people below me said, use 'ő' for people and use 'ez, az' for inanimate things :)
No, not adjective - it is still a conjuction, but a special one. 'Meg' is a so-called 'contrasting' or 'contrastive' (sorry I do not know the English term) conjuction, along with 'azonban', 'viszont' and 'pedig', which all stand AFTER what is to be contrasted, not BEFORE. In this sentence the 2nd 'ez' is to be contrasted aganst the 1st 'Ez':
Ez egy lány, ez meg egy fiú. Ez egy lány, ez pedig egy fiú. Ez egy lány, ez viszont egy fiú. Ez egy lány, ez azonban egy fiú.
See 2nd page bottom http://www.nyeomszsz.org/orszavak/pdf/ZimanyiElmeletKotoszok.pdf