Guppies are tiny tropical fish. Koi are very large cold water fish. You don't mention what species of catfish you mean, and there are hundreds. But simply from what I've pointed out you can see that they are incompatible. Only an idiot would try to do it.
It depends how big your Koi is and how big the Catfish is. If the catfish can fit in the Koi's mouth than it will get eaten.
a carp! go to www wikipedia.com and look up koi and koi is Japanese for carp! i got one! and they are cool! same as a plectomus! another name for a type of catfish! but tropical! a catfish is cold water (i think)!
Many aquarium specific and reptile/aquarium stores sell koi. Ask locally or do a search online to find a store near you.
The size of the koi will depend on many factors such as what type of koi it is, how much and what you feed it and the size of the aquarium or pond you keep it in.
Koi are a large growing fish (I've seen them at over 50cm long) and so they require a pond not an aquarium. They are also quite expensive. Koi are not for the beginner.
It depends on the catfish. if it is a small catfish ( 1-5 inches) sometimes the angelfish will try to eat it. if it is a medium sized catfish (6-9inches) you will be fine. BUT if you get a large catfish (10 inches and over) it will terrorize your angelfish by nipping at the angelfishes fins and if big enough will eat it. some of the catfish you can mix is a Cory catfish, Redtail catfish, upside down catfish and rubber lip catfish but that is all. Another thing is you should get the fish when they are young because angelfish are territorial. one last thing is that to mix the two fish you should get a tank 25 gallons or over or they will be too crammed and die even though 25 gallons is pretty large angelfish can grow up to 10 inches and all of the catfish that can mix can grow up to 7 inches
no
No! because tropical fish live in heated water and koi in cold water.
Koi have been bred/developed by the Japaneese to have colourfull, broad backs for viewing from on top as in a pond type situation. Koi grow very large and are not aquarium fish. If you really must have some kind of carp then get one of the fancy goldfish/shubunkin types.
Yes you can swim in it and they won't hurt you.
I feed mime freeze dried blood worms
Koi are pond fish and grow to over 24 inches or so when adult. It is unlikely they will breed for you in a small aquarium. (It is also unlikely they will live long in a small (under 75 gallons) aquarium)