no offense but i dont agree with you especially on the water changes 20 percent and also the fact females grow to 2.5 and if you stick by that rule its overstocked and you cant just do a measly 20 percent water change a week more like a 50. and not to mention the male guppies are probaly fancy and thier fins make there size larger too.
also guppies add significantly to the bioload , i think the best thing to do is 2 males 4 females
20% is enough in an established tank, if it was not enough for that specific tank, up to 25% or 30% if you change 50% then all your basically doing is more work than needed.
10 guppies in a 10G tank is absolutely fine with adequate filtering, you cant just say that because a fish as a longer fin it must have more room lol, long fin bristlenose are generally 4-5" but adding their fins your talking 8-9" you really saying this needs to be compensated for?
You talk about over stocking, well that can be argued from many angles, people give the 1" per gallon rule as a guide, not a "fact"
Many people, including myself when I was breeding, and even before I started breeding, would over filter tanks, compensate for over stocking, all you have to do is look at any lfs, at any point you go in, their guppies, mollies, platys, swordtails, endlers, grossidens, black chins, etc are all well crowded, this is constant in most popular lfs and they are healthy and in very well looked after tanks.
Its more about the filtration than the stocking, if you add 2 guppies per week to a 10G tank, there is no problem what so ever in taking it up to ten over the course of 5 weeks, the filtration will be more than capable of handling that, beneficial bacteria will colonise in your filter more and more as the loads get higher.
As I said, I went up to 20 in my 10G "I had about 6 tanks of them) all the water parameters check out spot on with all established filters (and they was running off sponge filters off an air pump too
"
If your ammonia and nitrite levels are fine with this load, there is nothing to worry about and no problem.
If you think a 10G tank is over stocked with 10 guppies, then no offence to you too, but, its a ridiculous statement.
Just to add, I told the op how I had my tanks so they had a insight to what others do, I am not telling them to do it, its simply information to whats possible.