Although so called 1" per Gallon is good guideline to follow as newbie, but not neceessarily the rule you have to follow. Idea is to give them comfortable/healthy environment.
I've seen people w/ understocked tank where fish are suffering but seen way overstocked tank where fish doing well. This all depend on keepers.
It seems you are doing well w/ overstocked tank (at least by some). And if I understood correctly, this used to house
JD and used same water which was well established.
Which ever size tank you go with, start off slowly. Maybe start off using water from 30 along with gravel, filters, Deco and same fish. Let this new tank stablized (cycled, if it does go thru minicycle) before adding more.
As far as numbers, it all depends how well one keeps up with routine maintenance. Well, you been doing well so far.
As far as aging water, I personally always aged water with heat, CIRCULATION/AERATION. I cut out the Dechlorinator use (but had one bottle just in case whole River system Crashed) since finding out how they work, that is chemically.
The problem rises since Chloramines maybe added to Tap water nowdays in which case need to used product to breakdown and detox. Can call your local Water Supplier for complete analysis of your tap. Test kits for Chlormaines are availbale but very expensive and not necessary, IMO, once analysis is obtained from supplier.
Good luck with new adventures and hoping to hear more!!