The thing about 7 Primes and 6 Non-Prime Original Transformers isn't explicitly mentioned in movie continuity, but it's a logical conclusion considering that:
+ Revenge of the Fallen continuity states that in the beginning, the AllSpark created 7 Primes. No other Transformers are mentioned as being created at the time (although it doesn't explicitly disclude them either).
+ The Original 13 Transformers are a multiversal singularity - thus the original 13 Transformers must exist in every Transformers universe.
+ Movie continuity has established that being a Prime is some kind of genetic heritage - i.e. only Transformers who are descendant from one of the original 7 Primes is a Prime; and bear the mark of the Prime somewhere on their body. Sentinel and Optimus are both from the lineage of the Primes.
+ Clearly not all Transformers are descendant of the Primes. In ROTF Optimus was supposedly the last of the Primes (after Sentinel Prime was presumed KIA) -- The Fallen claimed that only another Prime could threaten him and once Optimus was killed, he happily came to Earth, even though there were other Autobots present there. Clearly he didn't feel threatened by the other Autobots, presumably because they're not descendants of the Primes -- and presumably neither are any of the Decepticons, for if they were, The Fallen would probably have them killed too to eliminate them as potential threats to him.
+ In the movieverse comics, Megatron criticises the nepotism of having only Primes being their leaders. He sees himself as a more worthy successor to Sentinel Prime, even though he himself is not of the Prime dynasty.

Perhaps in your fanfic you could establish that each Prime is only able to reproduce once, whereas non-Prime Transformers have a higher "fertility rate"... because this would explain how in the beginning there were 7 Primes to 6 non-Primes, but then later on the number of Primes within the Cybertronian population became near extinct. If Primus intended for the Primes to be leaders, then it would stand to reason that he would limit their reproductive abilities in order to maintain balance amongst the Cybertronians.

But really, this is all looking at it purely from the movie-continuity POV. In all other Transformer universes being a Prime was never a genetic thing -- so if you're writing your thing to graft into the G1, G2 and BW continuity-family, I see no reason to even consider including any elements from the movie universe into it anyway.

13 Original Transformers = multiversal singularity, so you have to include it.
7 Original Primes = only in the movie universe. So feel free to ignore it if you want.