If the characters were elements from the PERIODIC TABLE.

There are definitely several that could work for each penguin, but here are the ones that first came to mind:

-- Skipper: Titanium: It's high-strength and corrosion-resistant and just sounds like something Skipper would want to be associated with.

-- Kowalski: Einsteinium: How could a scientist be represented 由 anything other than the element named after one of the world's most famous scientists? (Someone completely uninterested in Kowalski's science might also suggest boron as a way to imply his science is boring, however.)

-- Rico: Uranium: An unpredictable 企鹅 who loves explosions, how could Rico not be represented 由 one of the most well-known radioactive elements?

-- Private: Iron: Perhaps not the element 你 would first think of, but he is "Steel Penguin" after all, and steel is mostly made up of iron (and carbon, and sometimes other alloying materials, but I digress).

Skipper: Americium: Named after America, need I say more?

Kowalski: Nobelium: Named after (and discovered by) the Norwegian Nobel Institute, the same place where the Nobel Peace Prize recipients are announced; I’m sure many of whom are Kowalski’s idols.

Private: Carbon: Carbon, like Private, can bond with just about anything.

Rico: Lithium: One of the most reactive of the alkali metals, and highly unstable when not contained 由 something. It is also a metal used for particularly violent chemical reactions. Sounds like a good fit.

For Skipper, Francium; it's a highly reactive alkali metal (and nuclear). Kowalski would be Uranium (he's into the elements). Rico is so crazy that he's Ununoctium! Private is reactive when necessary; he's so Iron!