I have friends using the D5000 so, i know the 50mm fits. Albeit yes, it has to use some effort to press it against the mount, and also twisting it in place. I don't know what the situation was like at the shop, but it sounds like you were babying your camera and stopped the assistant from exerting more force than for normal lenses.
I could be wrong though. But that's what it sounds like to me.
I don't think you should be accepting a job from the CC when your understanding of the basics are limited as such.
But since it's the CC's decision in hiring you, it is up to them. =s
I learned DSLR basics from a D5000, and it probably shouldn't have any trouble shooting up to iso 1600, lens regardless.
Also, shooting on the 60mm @ 2.8f to allow for a faster shutterspeed will also thin down the usable DOF quite a bit.
Will such a depth of field render enough things in focus for your intended usage?
Would advise investing in an external flash instead...