Tanks for killies can be everything that can keep water. Plastic buckets, containers, rain collector, glass aquarium, pvc aquarium, you name it. Many of the keepers are using a lot 12L and 20L glass tanks when having many species. If you are going to have a lot of different killies I suggest you to chose a lot of the same tank sizes. This makes your possibilities much better if you have to build stands on a later stage. I use 63L glass aquariums which is shared in 3 room's each. I find this a good solution but others might not. It is up to you but remember that you will need about 3 aquariums for each strain that you want to have and breed. Of course one strain can do with one aquarium but for best breeding results it is the best to have the opportunity to separate the fry from the parents and again split up the fry in sex and size when they get a bit older. But it is indeed possible to raise fry in the parents tank but the result is often not that good.

One of my 63L shared in 3 killi tanks

For the smallest species about 10L and up should be fine for an adult pair. For new hatched fry you can use everything. Old ice cream containers and similar are perfect. When the fry is only days old you can have trouble to feed them in a 20L aquarium as the fry will not swim that much to find the food. If you give them the first days in a small container the chance for the fry to find the food is much bigger.

Just a little selection of my plastic gear ;-)

Many killies are great jumpers so it is a good idea to have a cover glass on your tank. If the killies gets scared or if the male is chasing the females the speed of the fish is so great that they can jump surprisingly far. Cover your tanks and close even small feeding holes to be sure to keep the fish inside.

Some killi keepers build whole rooms for their killies. My friend Bill Shenefelt is one of them. Have a look on his nice fishroom here.
Another friend Tim Addis is building houses for the tanks and killies. See how he does here.