What the LFS does with their fish and what we do in our home tanks for the long term health of the fish are very different things. The fish in the LFS tanks are in there for a short period of time, and the filteration systems are often centralised and overfiltered to handle the massive amounts of waste so the risk to the fish is minimal. 8 months is no time atall in fishkeeping terms, and I bet those guppies don't reach anywhere near their 18month to 2 year life span, or reach their maximum size of 2". I wouldn't be surprised if we get a thread from you in a couple of months time saying all my fish are dying! This is the same as when you see videos on youtube with tanks full to the brim with guppies or other fish, appalling especially considering your lack of concern given the response you've had on this thread.
Having 1" per litre instead of 1" per gallon is totally irresponsible, especially with an easily bred and active species such as guppies, but you've LOL'ed your way through this thread, and you don't want to listen so there we go.
Good luck, you're going to need it.