Align entire assembly to top plane?

Hi, is there any way I can shift the entire assembly to align to be parallel with the top plane? Every time I try through mating the assembly "explodes" and parts go everywhere. Also, is there any way I can fix the entire assembly togeghter so I cannot move individual parts, I have mated them but for some, concentric mates for example, I can still rotate the individual parts.

Thanks

SolidworksAssemblies