look at constraint but only in the X or Y etc

 

There's a Look At Constraint, in the Constraints tab, however if you only want this aim to work on the X, or maybe just the Y, you have to dig into the constraint, then go to the Common Tab (see top screenshot) and then only specify the rx or ry in the scope.

Remember to use Nulls to set custom pivots etc, then parent your desired geometry to the null.

using a transform that relies on $CEX $CEY $CEZ with a differently shaped/sized object

Use the julian matrix - (point wrangle)

matrix M=detail(1,"xform");

@P=@P*M;

plug the transform you want to "copy" into input 2 ("1") and the object you want to transform with said transform into input 1 ("0"). 

MAKE SURE the transform node has the xform ticked at the bottom, so the wrangle can actually access the "xform"

This is especially relevant for when you overuse $CEX etc bounding box for pivots & you suddenly change the geo size. Eg you rotate your thing into place, and do so using the centre of a short object, but suddenly you need to substitute it with a much taller thing.

group centroids

group centre/centroid - intstead of blasting out groups and using $CEX etc, you can just use $GCX, $GCY, $GCZ in your transform pivots.

How to move pivots in Houdini

press INSERT!