Okay so this is sounding good so far
Your taking it on board pretty well which is great!!
I think it might be worth trying the GT see how it goes but be quick to rehome if things go wrong!
Planning a tank when you want to upgrade is always hard - the problem is life happens, your car needs new brakes, your window breaks, a pipe bursts, your dog needs to go to the vet etc - these might not be applicable to you but you can see what I mean. The fish tank always gets put back - I have been there and it took me nearly a year longer than I anticipated... not a nice situation to be in and I had to rehome some fish.
For me in the 55 with serious intent to upgrade to around 90 gallons or more I would do
1 EBJD
2 Firemouths (unless this turned out to be a pair, then I would skip the EBJD or one of the FMs)
3 Swordtails (green natural morph, 1 male 2 female)
8 Blind Cave Tetras or Red Eye Tetras
3 Hoplo Catfish
that would be quite an accurate Central American tank.
And its a reasonable stock that you can build around in the next tank - so for instance you could look at adding more cichlids - Rainbow Cichlids would be great in there. Cutteri would be nice, Flyer Cichlid would work, Robertsoni would be nice if you could find them.
But you do have other options, there are a few species of Headstanders that would look great, some of the smaller plec species but not commons or similar sized rarer ones. If you dont mind mixing fish from other areas, some of the Synodontis Catfish would be nice. Wood catfish are always nice if you dont mind them hiding. Yoyo loaches and similar sized ones would work but not Clowns.
Something worth doing would be reading up on classifications of fish, so in your examples you went, Sharks, Rainbowfish,Catfish, Severums. Severums are a cichlid and dont really fit in with the above but could work... But equally listing things like Sharks and Catfish are quite vague so it might help to define which parts of these groups of fish you prefer. For example the sharks - things like Rainbow and Red Tailed Black Sharks would work (obviously as a singleton) but thinks like Bala Sharks would not and they are both very different fish classed as sharks by the hobby (though again obviously they are not lol). And with Catfish, do you prefer the active ones like Hoplos and Pictus or more secretive ones like Banjos and Humbugs - and then there are plecs....
But well done for coming to the conclusion of doing whats best for your fish
Its not always an easy decision but its always worth it, you were headed down a rough road with the tank and with the level of cost and commitment with these kind of hobbies you have to make sure your always going to enjoy it best you can
Wills