how can I use a brush CUSTOM re-topology? I am aware of the dynamic re-topology options in sculpting and I want to make it clear that what I want is slightly different. Is there a way to get something like a brush that when it paints over the mesh it tries to subdivide/merge tris in the least destructive way?

I think Sculptris had something like what I'm asking for but I've long since abandoned that software for many reasons, but I really miss that in blender.  My current work around is to zoom out with the clay brush, enable dyn-topo, then click.

While this works it doesn't allow for fine control, especially when you just want to merge a few tris.  Going into edit mode isn't feasible due to the lag from the shear number of polys generated while sculpting a complex mesh(for instance a model with 600,000 faces taking 5GB of memory), and it's tedious to do for every spot you want to clean up.  

Basically I want a brush that ONLY subdivides and collapses at an amount dependent on it's strength, NOT the brush size.  Can this be done in blender?  If so how?

Also please note that I have no experience modifying/adding brushes in blender so if that is a part of the answer please be patient with me and explain it to me like I'm a three y/o.  I have trouble following complicated answers.

I found a video showing the feature: https://www.youtube.com/watch?v=_Ia-fhpsabQ
Sorry about the quality, it's not my vid, but it I think if makes my question clearer.
Strenght Brush = 0 with Constant Detail

I usually take advantage of the Constant Detail dynotopology mode for this task. Basically the detail size does not depend anymore on the brush size on the screen, but it's a function of the Resolution parameter field.



Given that we can set up a brush that deoes nothing (set the Strenght to 0) but modify the topology with the set resolution. With the eyedropper we can easily pick up the resolution of the nearby faces like shown in the image sequence below.



You can restrict the action of the brush to Subdivide only or Collpase only if wanted. There's even the "Detail Flood Fill" button to automatically spread the same amount of resolution across the whole model.