Most cassettes are held together by three long screws. If you look closely, the heads are allen heads. I think they're like a 1 or a 1.5 metric. So yeah, they're pretty small. Stop by you LBS and ask for some spare spacers. They usually have a junk box with some old cassettes that they'll usually let you have to tear apart and get the spacers.
While there, ask them about a threaded axle for the rear hub. That way you will no longer have the fear of the rear wheel coming out. A new axle for a low end shimano hub shouldn't be more than about $15.00, and you sound mechanically inclined enought to swap it.
Good Luck & L8R
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