I have kept a wide range of Loaches and Cories in the same tanks and have never had a problem with Loaches attacking Cories or Angel Fish. Over the years the Cory population has dropped, but they never were attacked. Old age, I think. Here are a few combos:
Clown Loaches (young ones, about 3" long) are in a tank right now with an adult Angel and several other fish. The Loaches play around mostly near the bottom, and the Angel has found the low-flow areas of the tank near the top. I think there is one Cory in there, and several Bristlenose Cats. I got these guys when they were under 2" long, and they have grown really fast. (Clown Loaches did not bother them, even when they were little) The Loaches do not bother any of the fish in this tank. Mostly they just hang out with each other.
B. kubotai with several cats including Rineloricaria and a lone Cory. There is at least one sidthemunki in this tank, too. Larger fish include Rainbows and Clown Barbs.
Sidthemunkis with some Shell Dwelling Cichlids, and 2 Loricariads; one Peckoltia and one Pleco that is not a common, but is pretty large. (Moved to the next tank)
Dojo Loach with Roseline Barbs, an Oto and the Plec from the previous set up. There is an Oto in this tank, too.
Clown Loaches, Bichers and some others with Corydorus barbatus. The Bichers chased the Cories out of this tank, I found them in the sump. Bichers are predatory fish, and will eat any other fish that fits in their mouth, and they have big mouths.
Kuhlie Loaches with Guppies and Bronze Cories. I think the Bronze Cories are past breeding age, but they are parents and some of the offspring.
B. striata with Endlers Livebearers. Maybe the Zebra Loaches were eating Endler's fry, but they never bothered the adults.
The only Loach vs Angel problem was the other way around: The Angels defended their nest, eggs and fry against all comers, and the Loaches (sids) just went and hid under the substrate. I had to take down that whole tank to find the little guys.
The only Loaches that I have had chase the other fish were Skunk Loaches. I have heard that Tiger Loaches (Synocrossus) can be pretty aggressive, too.
New fish, any species, needs to be quarantined, and treated for whatever parasite or disease shows up before they are placed in the display tank. SOme species are known for carrying certain diseases or parasites, so treating for that is usually done, even if the fish show no symptoms. Many bottom feeders (Cats, Loaches, others) can pick up internal parasites quite easily, and if you do not quarantine and treat for worms these can get passed along to the fish in the main tank.
No matter how nice a species is in general, there is always a range of behavior, and one individual fish might be uncharacteristically aggressive. Loaches, Cichlids and Bettas are the fish that are most often thought of when anyone mentions the idea, "But each fish is different, watch them..."
38 tanks, 2 ponds over 4000 liters of water to keep clean and fresh.
Happy fish keeping!