when is it necessary to have a continuous mesh over separate pieces? Everywhere I look people say that animated models need to be one continuous mesh with quads. A lot of tutorials of people who are doing hard surface modeling via poly modeling vs sculpting often model there final object with several different pieces. Like if it was a helmet with rivets they'll model the rivets separate and just leave them on top. If you want to animated the helmet on a figures head do you have to join all of it into a solid mesh? if you do that do you re-topology it the same approach as a sculpt? 

I ask in reference to a medieval great helm I'm modeling. it would be easier to model the rivets, and the cross (separate pieces that is riveted around the eyes and seams. I want to be able to 3d print it and but it on a character to be animated. Does the entire helmet need to be one solid mesh? I don't want to sculpt it.
To be clear: First you say 


  Everywhere I look people say that animated models need to be one continuous mesh with quads.


Then you say


  A lot of tutorials of people who are doing hard surface modeling via poly modeling vs sculpting often model there final object with several different pieces.


Simply understand that it's not necessarily a contradiction for people to be saying these two different things, because not all modeling projects have the same goal. I suspect that many of those you've been hearing are simply making assumptions about what other people are going to be doing with their models, and so maybe they are not entirely clear about their own goals when explaining their methods, so that others will understand what those methods are best suited to accomplish. Sometimes we take too much for granted.

Hard surface modeling can use various separated parts, just as you say--and even boolean them together without worry--because the end result will not need deformations. When you hear people say that models need to be one continuous mesh with quads, they are likely assuming the goal of organic modeling, such as for characters. 

Once this is understood, the course becomes rather clear, and there's no need for concern. For a helmet, you could probably do whatever you wanted. The only real issue I can foresee might arise from the fact that you want this model for 3D printing. That is going to require you to take more care than might otherwise be necessary, but I can't say in exactly what way, as I am not an expert on modeling for printing purposes. It might be a good idea to research this topic specifically, since the cost of physical material is at stake.