Re-decorate my 55g tank

Well, in regards to the angels - they will do fine in a 55gal tank. I do not know if they will get along with the Severums, but since they are both cichlids I'd be careful. The silver dollars are high-end plant eaters, meaning they will demolish anything green and not plastic in your tank. If you want to house them you will need a school of at least 5, preferably more, and will need fake plants for decor. I know that angels and silver dollars prefer a 'planted' (fake plants) environment, with driftwood, plants for cover, and a sand substrate. You will want to overfilter, as all of these fish get fairly large.

Other fish that can go in with the angels and dollars are the following:

Cories, Serpea Tetra, Black Skirt Tetra, most dwarf gouramis, all rainbowfish, and most livebearers. Note that black skirts may harass the angelfish, and that you should avoid any gouramis under 1in long with adult angelfish, as the angels will eat anything that fits into their mouth. Rainbowfish need space to swim, and will do well in that tank. Livebearers will breed their way into overpopulating, so if possible only get male individuals or just skip them entirely.

Corydoras generally like to be in groups. A single pair will likely be very nervous and shy. I'd say you'd be better off with at least six of the same species.

Pleco will depend on the species. There are many different varieties of pleco available. Some of them (the "common" pleco in particular) are known for eating the slime coat off of other fish. Especially flat-bodied fish like angels and silver dollars. I think rubberlip plecos and bristlenose plecos are generally considered safe in that regard. They also stay relatively small (6-ish inches) as opposed to high-fins or commons which can grow to a foot long or even two feet in the case of the latter!

If you're going to get two gouramis, just make sure that you don't get two males as they tend to become aggressive toward each other. A male-female pair would be suitable for some species of gourami but, again, depending on the exact variety you're planning on getting, you may find it's better to get a trio (one male and two females). Sometimes a male can be hard on a lone female and it helps if he can divide his attentions between two.

Silver dollars are schoolers so go with six and two angels.