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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.