gallons per fish

Hi all,
Just found this group, so I'm a real newbie here and pretty much new to
goldfish keeping. Got into goldfish by accident thanks to my darling
daughter-she brought home some fish used for decorations at her high school
prom and after 4 of 6 died in the little fish bowl she found I went out and
bought a 10 gal tank, filter ,etc and the two remaining fish seemed to do
ok. Then she came home with two more from a church carnival (all were
"feeder" fish) that I put in the tank. One died right away, but the other
was fine, except for attacking the two original fish (hence the current name
of PF for psycho fish). The two original fish eventually died from fin rot
due to these attacks (I'm assumong that's what it was based on info from
various websites). So now I have one fish in a 10 gal tank. Didn't want him
(her?) to be lonely so,after spending $40 on a water testing kit (tested
OK), went to the loacl Petco and bought a red capped oranda and a fantail.
All this happened about 7 months ago and so far they're doing fine (PF still
chases the oranda alot but doesn't seem to be doing any damage). Everthing
I've read on various websites has said I need 10 gal per fish to properly
care for my babies, yet the three of them seem to be doing fine in my little
10 gal tank. I feed them twice a day (presoaked flakes or
pellets,peas,romaine lettuce,brine shrimp), test the water ever two weeks
( 0 ammonia, 0 nitrites, nitrates a little high), add water when the level
drops a inch or so from the top of the tank (never done the "change 25% of
the water every week"), change the filter media every month or so, and
vacuum the gravel every now and then. So my question is- am I just lucky or
does the 10 gal per fish refer to larger fish than I have (they have grown
quite a bit-body length is now about 3")? I'd like to get a couple more
fish so I've been looking at new tanks (a 36 gal bowfront) but I'm worried
that at 10 gal per fish it won't be big enough to support 5 fish. Any advice
would be greatly appreciated!
Thnaks in advance!
ken