Definitely check out TidyPlates, but I think the problem you describe is fixable without TidyPlates. You can change the way nameplates display/move/stack within the interface panel. It sounds like maybe you have yours set to spread? Stacking with prevent them from moving around a whole lot, but if have too many targets they'll move off the screen eventually.I had an absolute bitch of a time switching targets on that kill; I really think I need to check out tidyplates because having them move around makes it difficult to acquire targets and mouse-over targeting doesn't work. The other annoying thing that happened is that I was unable to re-designate Garrosh as my focus target after he came out of T1, and didn't realize it.
TidyPlates is more for displaying auras on the nameplates or for better display of castbars, health percentages/absolutes, threat, etc.