Plaster/Metal Lath Questions

I installed an invisible fence for a friend, and used up the only receptacle in her garage; the other one had an extension cord going to her garage door opener.
I replaced the outlet with a GFCI and ran romex to the GDO and put in a box on the ceiling. My only problem was getting the cable staples to stay in the ceiling. It was all plaster with metal lath, so my stud finder didn't work. I expected there to be studs at the corners (there were two things, presumably steel beams, boxed in), but there didn't seem to be. I put in about twice the number of staples I normally would, and it seems to be stable, but there has to be a better way to fasten the romex to the ceiling. Any suggestions?
Also, there are a few holes and cracks in the plaster. Since is presumably is a fire wall between the garage and the bedrooms above, they really should be patched. What would the proper materials/methods for this be. I have no experience at plastering, but it can't be too hard to do a few patches. A reference to something on the internet would be great.
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