In obtaining concrete for a slab in a hydronic system in an already enclosed basement, there are several additives in concrete that have fairly low costs. How much additional benefit is there for having plasticizer added to the concrete to allow for lower water content, thus improving the curing process? This was recommended by the concrete supplier. Secondly, my foundation walls have an additive by Kryton that actually does a heck of a job in terms of waterproofing my walls (the cold joints are another story, but they will be taken care of). Is the use of such of a additive (a crystal of sorts) that would limit water permeability of the slab be of any benefit? Or would it be unnecessary since the slab itself is protected by vapor barrier and not realistically have any water issues anyhow?

+1 for the plasticizer additive...it will reduce cracking and perhaps even result in stronger slab by reducing risk of water loss.

0 for waterproofing additive. It likely wouldn't hurt, but it provides little additional benefit if you have a true VB below the slab (i.e., not just EPS/XPS).

Less water content (enabled by the work-ability provided by a plasticizer/water reducer) means stronger concrete . But don't count on that for anything - still plan for cracks.

Thanks for the feedback! That's basically the direction I was thinking.

jonr - I appreciate the saying that has been floated around - 2 types of concrete.... that which has cracked, and that which will crack.

Plasticizer is a water reducer , so yes that it will make stronger concrete but the main reason it makes stronger concrete is the fact that it makes the concrete "more plastic." In this it acts as a densifier. There is less air gaps between aggregates and between the aggregates and any rebar (or Pex). Because it is denser, it is stronger and also less porous.
And of course it makes placing much easier. But check with your supplier. The plasticizer we use has a time limit of between 20 and 40 minutes, then it reverts to the original slump. (great for placing in ICF.) Some water reducers do not kick back but remain at their specified slump for the duration. The first needs to be placed in the mixer at the site. The second can be place in the mixer at the mill.
And no, plasticizers do not add plastic or vinyl to the mix!

The best way to avoid unexpected cracks is to properly grade the base. Shrink cracks will always choose the weakest link (hopefully the crack inducer.) If there is a thinner spot in the slab, it is the place where the crack will usually start. If your slab is of uniform thickness over a properly compacted base, you greatly reduce the chance of cracks.

This is a good concrete resource tool:

NRMCA