There are ceramics which are ok in the tank but some contain calcerous substances which will increase the pH level.
Glass ornaments are fine.
And no, painted decors should not be used in the tank as the paints will easily come off and contaminate the tank.
But those things are unnatural in the tank.
I'd settle with rocks, bogwoods(should be boiled to leach out tannins which will reduce the pH level) and live plants(on 2nd thought, your fish will eat them
By the way, on the issue of your fish, you'll need a 100 gallons to accommodate them. Oscars are fast growers and will reach 12 inches potential size. If your plec is a common plec(Hypostomus plecostomus), then it will also outgrow your tank by 14 inches.
And so will the blood parrots and goldfish when they reach 8 inches.
I'd say you return them to the lfs and get small shoaling fish instead. Harlequin rasboras(Rasbora heteromorpha) are ok in your tank. 15 will fit in the tank. For bottom, get an ancistrus(bristlenose) to replace your plec(if it was really a common plec).