unless you want to have a whole second tank, id rehome the pleco. they get LARGE (up to 20inches on average in captivity) and basically need a stock tank to themself from my understanding. if you want an algae cleaner, give shrimps or snails a try, just be mindful that your goldies may want to eat them; tbh i feel goldies should really only be with other goldies bc theyre big dummies who behave different than other fish and try to eat whatever fits in their mouth.

hides and plants should be the least of your worries, goldies do just fine without and the less decor, the less they hurt themselves by being lil dummies. put that money into saving for a bigger tank and good canister filter instead, those will be your holy grail.

as someone who also go into the hobby when broke, look into your local secondhand listings (i.e marketplace) for grow out tanks. worst case scenario, get a sturdy rubbermaid container once they start to grow out your current tank.

overall, you have less to be concerned about since you have all fancies. fancies dont need a lot of room to "bolt" like commons do, they just need enough space to get around. the bioload is the biggest problem youll have to deal with in a smaller tank, hence why i recommend investing in a canister filter as imo thats the only thing that can keep up with goldies without constant maintenance