If your swordtail is male, he is probably inseminating your platys. If the sword is a female, the male platys are probably trying to imseminate her. Their offspring could be interesting, maybe intermediate.

As mentioned, goldie is an omnivore and will defecate a lot. (Poop happens, more so in the goldfish's case.) Commercial goldfish food is unusual in that it not only is a good part of it veggie material, but also there are starch items not usually found in fish foods.

They are cold water fish and usually need a cooling cycle in the winter season. They are also Cyprinids. In their second or third year (or when about 4 inches/ 10 cm) they may have grown enough to be ready to spawn - if they had a cool season. You may want to get goldy a couple of companions (4-6) so that there will be both males and females. Your bowl for them should hold 20-55 gallons (75-210 liters) or more! I would recommend tanks.

Given good care, they can live for 10-25 years. (Just figure out what age we will be and where we will be 20 years from now.) Most never live very long because they come from the farm carrying a lot of parasites. Many also perish because of newbie care and bowls which are far, far too small for them to live healthy and happy lives in. And then new gold fish keepers ask, "What water changes?" :(

If those guys sound long lived, koi (the colored pond carp) have lived over a 100 years! We seldom think about how long a fish can live when we shop for them.

When the spring warms their waters up to the 60s or 70s (16-23 C) the males will develop tubercles, (which sounds better than pimples) which are evidentially contact organs. They may gang spawn and will produce 100s or 10,000s of eggs. A bunch of plants or spawning mops will provide surface for them to settle on. Parents are egg eaters, but in nature some eggs will survive. You could remove the parents (the tank will need a lot of water changes because of the left over materials from the spawning anyway) or the eggs can be removed and put in a large tank, vat or pond. In a week or so, the healthy eggs will hatch. Grazing in a pond, supplemented with newly hatched baby brine shrimp (bbs) and tiny pellet food  especially designed for them and those sliced veggies will serve them well.

Goldfish and weather loaches are spawned commercially in warm climates. I don't know if "they" have found a way to get those temperate zone fish around a cold spell or whether they just inject them with hormones.

Your sucking catfish probably belongs to a large group of South American fishes called Loricariids or family Loricariidae. There are over 1,000 species in that group. Yours, if (pond) bred in commercial channels, may be a member of the genus Hypostomus. Plecostomus is a genuine species name, but is used in a general sense for a lot of other fishes.

Most are vegetarians and a lot starve because they are incorrectly fed. They might actually do better on the goldfish flakes than the other stuff. Also drop them spirulina tablets, slices of zucchini or cucumber. If they have a hard time eating those veggies, they could be lightly blanched on the stove or left in the microwave a minute. Those slices should then sink. Some people just run a new steel nail or screw though them and let them sink.

They can get over a foot/ 30 cm long, if it is one of the species I'm guessing. ALL OF US have bought fish, unaware of their eventual size, foods, or life spans. A sign of a person maturing in the aquarist's craft is that they will look for those things. And they will still be guilty of an occasional impulse purchase. ;)