I wouldn't say Caesar is an experienced fighter, he just happens to have worked in the New World in recent years, not all experienced fighters develop skills with Haki, but in the New World it's a must if you're involved in many fights (such as the Donquixote Leaders, especially Diamante whom operates and fights with lots of Colosseum gladiators/contestants)Caesar is an experienced fighter and lives in the New World. So is Monet. Their Haki is where again? There's also Ace who showed no signs of Haki either. I'm just not into the whole giving abilities we've never seen people use thing unless their portrayal demands it, and Diamante's doesn't.
Ace showed no signs of Haki? (What about his Haoshoku Haki!?) Dream on... Oda just didn't know how to portray it yet,
giving probable abilities which justify the strength of a character isn't unreasonable. I can't imagine it ever being possible (ever) to be stronger than Franky in his current state without Haki, Franky probably has basic skills with Haki (kinda like Tashigi) as well so...