For a cockatiel, the minimum recommendation cage size is 20 inch wide, 20 inch deep and 28 inch high.
to spread there wings often when they don't get to fly around or don't have a big enough cage.
Cockatiels are very territorial and are okay either by themselves or one other but I don't know about seven. For one cockatiel you at least need a cage were it can stretch out it's wings and fly around in so for seven, get an aviary cage.
Cockatiels do not need anything to have a mate. Just put him/her in the cage with your cockatiel, however, if you want them to mate, they will not mate if you do not have a nesting box. In order to breed cockatiels, you will need a nesting box.
About as big as a great danes cage i would say
Cockatiels do not typically have a strong or unpleasant odor. Regular cleaning of their cage and providing a healthy diet can help keep them smelling fresh.
8.5ft
Really depends on how big the bird is / how big it will get. The bigger the bird, the bigger the cage you will need. I would recommend getting at least a 20Lx20Wx29H sized cage for a parrot.
you would probably need a medium sized cage for a male rat as they are quite big. but you could get a cage with a few ladders and and a exercise wheel to keep it active.
you shouldn't cage dolphins dolphins need relaxing environments, also if you picked out a normal cage like a dog cage it (obviously) wouldn't keep water in
yes you can. cockatiels enjoy human company as well as cockatiels or other birds company as well as the voice of other birds, just make sure that the cage you keep them in is big enough and doesn't crowd them--------------------------------------------------each cockatiel is different and males / females react different. try putting the cages close to first see if one gets aggressive. i had times where they would attack each other badly due to territory dominance.
Cockatiels eat paper to keep themselves entertained. Usually they don't actually eat it, they just tear it up and chew on it. If you are uncomfortable with your cockatiels eating paper, you can try to distract them from the habit. Putting more toys in the cage might help.
No it's fine to feed cockatiels in their cage. I feed mine in their cage as they will not eat out of my hand. Just make sure you clean their cage thoroughly every day, including perches, dishes, toys etc. Also make sure your birds have vegetables and fruit to eat aswell as seeds or pellets to keep them healthy.