Ikari Warriors are in KOF? I've never touched a KOF game in my life.....
....
Are they playable characters?
Different names (although there were like three Ikari games, so maybe one of them uses "Ralf and Clark," I dunno), but see Mox Maniac's post. Ralf and Clark are the Ikari warriors. Heidern is their commanding officer. Leona is Heidern's adopted daughter, too. And while it's not Ikari, Ralf and Clark appear as playable characters in Metal Slug 6, returning to their roots. There's also some Metal Slug mobile phone games with Leona.
The original KOF '94 really was an "SNK all-stars" game. Fatal Fury and Art Of Fighting are the most recognizable, but then you have the Ikari team, and Team China is actually the Psycho Solider team. Psycho Soldier is an older SNK arcade game, and is where KOF's Athena (not to be confused with the other Athena character SNK has) and Kensou come from.
You can use an Ikari-style Ralf (his look) in the form of Another Ralf in the KOF Maximum Impact games.
Kinda hard to see in this pic, but it's all I have.