Haibara x Akai: They could have known each other since he was her sister's boyfriend. Akai hints that he doesn't want Ai to see him yet (ep 342) when she was unconscious.
They have similar personalities. Both have a tragic past and lost someone dear to them (Akemi). They have a mysterious, calm attitude that gives a dark impression. They are cynical heroes who care about the people that they love and are always ready to give away their lives.
Haibara x Okiya: If Okiya wasn't Akai in disguise. He seems interested in Haibara. Okiya was shown saving Haibara from falling in the water (ep 542), a gesture that he wants to be her hero. He's also a mysterious personality, but gives a brighter impression than Akai...his kindness makes a good contrast with Haibara's sarcasm... His optimism completes Haibara's cynical attitude.
We don't know Okiya enough yet, but his character so far fits well with Haibara. And Akai...I think he would like a more cheerful girl like Akemi or Ran...but his appearance with Haibara resembles a gorgeous gothic couple from vampire fiction...so they make me drool.
EDIT: Not to mention that the strange vibes that Haibara senses from Okiya ("he could be a Black org member but he's not so bad" feeling) could mean that he's Akai...and she's just attracted to him.





these two guy are the only who eventually can fit with shiho and her personality


So you're saying all they need is time? That is easy in fiction, maybe Okiya and adult Shiho will meet in the end and we get a happily ever after. Or maybe a 1 year time lapse into the next chapter. It won't be that hard. And yeah Conan took a while to break Haibara's shell...but Conan is a bit harsh (he screamed at her the first time they met and called her a murderer...). Like they say, you might spend years hitting a rock and then only a drop of water can break it open. So maybe Shiho would show her real self easily with Okiya because he has a kinder/ softer attitude than Conan.