There is no official collective noun for the platypus animal because this mammal is typically a solitary creature, although a group of platypuses could be referred to as a "paddle." Collective nouns are usually created for animals that commonly appear in groups, such as a pride of lions or a herd of cattle.
There is also no official word used to describe a baby platypus. This is perhaps because when the baby platypus first emerges in the wild, they are rarely spotted by people. They can also be very difficult to distinguish from adult-sized platypuses because they will have no obvious distinctions in their anatomy that would distinguish them as being in the adult stage. At just 3 or 4 months old, a baby platypus has all of its features and fur fully grown and defined — they are simply a bit smaller. However, some zookeepers have come to call baby platypuses "puggles." This is a generic term for any animal that is in its baby stage, though, so it is not specifically meant to describe baby platypuses. As for using the plural form of "platypus," the word that is used is "platypuses." However, "anthropodes" could also be used, according to the Australian Platypus Conservatory, from the Greek roots of the word, but this term is not popularly used.