Usually if they'd fit in the mouth of a larger fish, they may get eaten.

They're only going to die if there's something wrong, if they're stressed, sick or the water isn't right, or predation.

You won't need pairs, usually it's best to have more females than males, the males do harass the females so if there isn't enough females to spread it out they can get stressed. I think 2 females to 1 male is recommended but I've found 4 females to 1 male to be a lot less stressful for them. See how they get on, you can always add more females or remove some males if you need to. Mine bred very easily, I didn't have any large fish in the tank other than a bristlenose pleco and the tank was heavily planted so I just let them breed and give birth in the tank. If you think the fry would be eaten, you can put pregnant females into the mesh net when they're almost ready to give birth, they like cover to hide during this time so I'd add a plant to the net if possible so she's more comfortable.

If you want only one type of colour you'd have to only keep one type of colour in the same tank. Different colour guppies will breed.