I think I have used both in the past. Generally speaking there is more pigment in intaglio inks, since those are usually used in thinner layers than relief inks, but I cannot guarantee that this is the case with these inks.

Don’t add regular linseed oil (from oil painting stuff) to the inks. It will soak to the paper and will cause oily halo around the ink. Printmaking oils are much stiffer stuff. Tack reducer may work to achieve what you are looking for, or add burnt plate oil (or the equivalent safe wash oil) to the ink to reduce the viscosity. I would also try using heat plate to increase the ink temperature and also stir the ink heavily, which are tradtional methods for making the ink easier to work with.

Almost anything works in monotype. Drypoint is more demanding for ink and you will have to try and see if it works. My gut feeling is that a relief ink with tack reducer might be too ‘short’ for drypoint, but that is just my personal preference.