Sarge didn't have to do a deal he didn't want to do
No, but arguably it is a deal that he wants to do. Sarge is not escaping Shield's custody anytime soon. Sarge's main hope of escape was Jaco, but Daisy shut that attempt down quite effectively. So, if you're Sarge then making a trade for your freedom and the freedom of your team for the information that the cold will shut down the Shrike is a win-win scenario for Sarge. He might get his freedom, but more importantly he definitely shuts down a Shrike problem.
Even if Sarge loses on the personal freedom question, it's still a win on the "containing the Shrike" problem from his perspective. Having said that if Mac doesn't uphold his end of the deal then Sarge is less likely to co-operate in the future. Although, generally, you have to factor in that Sarge doesn't give a crap about Earth and he could move on to the next planet just fine - even though, in the game of chicken, he won't.
And how come so many Shrikes invade Earth ?
In episode 1 of the season, in the section where Jaco arrives on Earth through the portal, then a flock of Shrikes fly up and out of the portal before Jaco arrives. The dialogue in episode one suggests that the spatial distortions have existed for several months beforehand. So, in terms of how many Shrikes might be on Earth - I think we could be looking at anywhere between a few hundred and up to a thousand. There's probably some critical number required to detonate the planet. That'll probably be explained in a future episode.
FitzSimmons being rescued by someone looking for a crew to help get to earth to recover stolen artifacts.
At this point that feels more like a writing excuse to get FitzSimmons back to Earth. However, you could ask: what are the artefacts and who were they stolen from? The trite answers are the Shrike and their employer. The more complicated answer is some other artefact and the Shrike's employer.
Although raises the question of why they're so murderous when their apparent raison d'etre is saving worlds.
If they're there to save worlds then they do seem to be rather bad at their job. However, I don't think they're there to save worlds. Rather, they're there to introduce the next villain to the MCU in a dramatic style after the next film is released. Much like the show was in season 1, where they couldn't reveal Hydra as a villain until the film was released that showed it. You may say that I'm a cynic ... you would be right.