Actually, I do not find that system to be outdated at all.Hello,
Tank & "stuff": 20L, 20 lbs Eco Complete, Azoo sponge filter rated for 30 gallons, Marineland bio-wheel 100 rated for 20 gallons (w/ sponge over the intake) , with 2 30" Finnex Planted + lights over a very tight glass lid.
Plants: 4 amazon swords various sizes from fairly large to pretty small, 6 Crypts from fairly large to fairly small, a few sprigs of dwarf sag left over from another tank, a 2"x4" ss mesh of 2-4 inch long peacock moss, a dinner plate sized area of floating water sprite, and 1 small (4 leaf) java fern.
Decorations: 1 1.5' long new piece of what I believe is malaysian driftwood and a smaller previously used piece of driftwood.
Water: 0 Ammonia, 0 Nitrites, 0 Nitrates, kH 8, GH 8, pH 7.5 (slowing working this down)
I wanted blackwater so with the new driftwood, 3 IALs and a half cup of peat in a stocking toe + dosing Kent Blackwater Extract I have very blackwater. I'm also dosing Excel and Flourish.
Currently Stocked: 10 Neon Tetra, 5 Cory Habrosus, 3 Oto vestitus
I want to add 2 Apisto Agassizii (a pair), 8 Marble Hatchets, and 1 rare-ish smaller max sized pleco (5"-6")
I know this excedes the 1" of fish per gallon rule that's kinda out dated but the tank is over filtered, nicely planted and I'm doing weekly 20% water changes.
So would this be overstocked, packet to the maximum allowable limit, or would I have room to grow?
Yes, we have come a long way since that rule was made. And it isn't perfect. Some fish produce a higher bioload than others. Calculate that with there is not actually 20 gallons of water in there after everything else.
I think you are stocked right at the point of being full per recommendations.
I honestly would suggest starting another tank.
I was at the same point you are, and I wanted to add more fish to my 30 gallon. So I started a 10 gallon and moved some of my fish to it.