Depth = front to back. But just from those measurements I can say it's an average/slightly taller than average 60l ish tank. So it's definitely 55l not 55 gal.
Now we know that I'd say the following about your stock...
2 blushing koi angelfish
These aren't suitable for the height of the tank. When kept in tanks that are too shallow they become deformed on their bottom half so instead of staying the nice triangular shape they become a bit more square.
It also might not be big enough to house them both together long term as they could become terrirotial to each other.
2 dwarf gouramis
Male or female? I imagine you might have problems with these squabbling sometimes. But as for the tank size they're fine to stay long term.
2 albino koi swordtails
I'm personally against putting male swords into a tank any less than 2.5foot and 3ft is the preferred due to the long sword and the fact they're very territorial/agressive when fully grow. If they're females though I can't see you having any problems and they will be happy in there long term.
1 neon platy
Male or female, should be fine in the tank.
1 threadfin rainbow
Edit: oops, scrap everything previously written here... not quite sure what species of rainbow I was thinking of... anyways. Yes the 2ft tank is fine for them. But they do need to be in a larger group of their own species really... however I wouldn't suggest adding anymore yet as there just isn't room to add more fish.
5 bristlenose plecs
This is alot of plec for such a small tank. I personally don't like bristles in 60l as I think compared to the size of the tank they look massive. But plenty of people make it work. Still I definitely would say only keep 1 or 2 VERY max. 5 is far too many and they do produce a large amount of waste meaning you could run into water quality issues in the future. Problem is you probably wouldn't know about the water quality issues until the fish started gasping/dying. Not a situation you want to be in.
1 black-phantom tetra
Is a shoaling fish and as such should really be in a group of it's own species. As with the rainbows 5+ is recommended. However I wouldn't recommend adding anymore fish yet.
1 guppy
Should be fine in that size tank
Overall I'd say you're significantly overstocked and heading towards trouble in the future. I would say you're overstocked (going by adult fish sizes) and that you're definitely fully stocked even going by current fishes sizes.
I can't suggest adding anymore fish unless you can adjust the current stock list.
All the tetras, the Rainbow and live bearers do need to be in groups of 6+,
I'd have to disagree that the livebearers need to be in groups... they don't have issues with being kept as singles...
I'd advise rehoming the Angels as the tank is too small for them, and they will get stunted (if they have been in there for 4 years they already will be)
The angels are only 5-6cm and so have definitely not been in there for 4 years and wont have been stunted yet. That's why I asked for rough fish sizes. Aswell as trying to estimate how close 'future trouble' will be.