Yeah, we're looking at more like, 100-150 years of integration. Even then its going to be a powder keg due to the sheer bad blood over the whole genocide thing. Because remember, that's what the Aliens were going to do to humanity - commit genocide and turn the human goop into material to make better alien-human hybrids for the Ethereals to inhabit. Any human who's fine with that after even 20 years is a bloody idiot with the survival instinct of a particularly depressed lemming.
Everyone is going on about how humans are going to oppress the aliens, but you'd think there'd be some die-hard Ethereal loyalists among the alien ranks too, or aliens who want their time in the sun now that the Ethereals are gone.
Again, if you want a better story about Alien-human integration serving as a thinly veiled allegory on race relations, go watch Alien Nation. This is just crap.
When Volk and the Reapers show up in WOTC, Volk's basically echoing that sentiment. Then XCOM and the Skirmishers are like, 'yeah, but we have these strong Chosen guys to deal with now.' And then Volk basically caves and agrees to help, no more questions asked. Even if he had been arm-twisted into joining up, the resentment and factional distrust between the Reapers and the Skirmishers would probably have made Bradford send them on separate missions. But in the game it's like all is forgiven and everyone joins the Rainbow Coalition. To say it strains the law of probability is an understatement.
It actually could have made for some interesting cloak-and-dagger type of missions and story paths. Mission gets borked, Reapers blame Skirmishers. Skirmishers disappear for some time and Reapers demand that XCOM investigates, something like that. But instead it's big happy family and everyone gets their new OP super kill DLC units. After that initial dust-up they are part of the game, but no longer part of the story.