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 Von 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 pinguin who loves explosions, how could Rico not be represented Von one of the most well-known radioactive elements?

-- Private: Iron: Perhaps not the element Du 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 Von 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!