The people who the World Government deems powerful enough to stand between the most powerful criminals on the planet and the rest of the world. Magellan seems qualified, and Shiryu probably was as well. But nobody else seems qualified.
I'd say the 3 Demon Guards are qualified to deal with all but the most ridiculous prisoners, which is all you should expect.

End of the day, the staff of Impel Down wouldn't even be able to contain two Level 6 prisoners escaping simultaneously. Because Magellan would only be able to fight one while the other would end up soloing the entire rest of the facility.
Since Level 6 Prisoners include people like the Shichibukai, and people dangerous enough to have been erased from history, so... Uhhh, no shit? Do you really expect Impel Down to have enough power to rival one of the 3 Great Powers?

Also, who is to say that Magellen couldn't take on two at the same time?