Janitor fishes are the biggies in the tank. But big as they are they have serene dispositions and are nocturnal creatures, so you'll see them active in the night. I guess that they can be joined with the small ones because the small fishes can easily fit in the big tanks. I'm not sure though if both can cohabitate.

I assume you mean plecostamus species. It depends somewhat on the species. Common plecos get quite large, and they become more omnivorous as they grown. So when they get big enough to fit fish into their mouth opening, which is relatively small, they will eat small fish. If you stick to a dwarf species like the rubberlip, it will never get big enough to eat an adult platy but I couldn't rule out thinning out the fry a little.

They help clean the tank by eating those dirt that the main fishes did not eat. But I think they also make the tank dirty because of their poop. But I guess they can help the tank more than giving it a negative thing.

Janitor fish eats smaller fish and eggs. You just have to make sure you put bigger fish together with a janitor fish. Janitor fish are pretty cool actually.

I think molly is too small and not safe for janitor fish. Janitor fish will kill your fish, and actually I don't like the face of janitor fish.

Janitor fish are invasive so if you think about having them as pet, i would not recommend it. If your objective is help clean tanks better, i think it's more wise to have a more efficient filtering system.

Yes you can keep the with Mollies and Platys, and yes they clean the tank very well, they eat a lot of food that the other fish don't eat, it depends on your fish tank volume but I think that two janitor fish is enough.

Absolutely not. Janitor fish is a predator to small fishes, so if you put it on a tank filled with small fishes they could just be easily wipe off the face of the earth when they are hungry.Thus, it is helpful in cleaning up the tank algae, debris but it still not advisable in keeping up company with small fishes like mollies and platys.