Felix is not 28 years old i know that much. he is probably about 16 or something (lol from the kiwi contest with felix).

anyway, was it 20 years it took for SoF to get to UNSC controlled space? if not, why should Chief be at the ark? I'm probably wrong but

Chief couldnt have met the SoF before halo CE because then he would know of the flood... Yea there is probably many many more reasons for why SoF could not have met Chief.