This is just a suggestion, you can do whatever you wish.
Find out what kind of milk the kefir grains were grown in and use that milk... at least to start with.
When you have a back up of grains, then you can play with them with other milks.
I keep my kefir grain backup in the refrigerator covered in milk which I change out about once a month (when I think about it) - I drink the milk I pour off when I make the change to fresh milk for the backup grains. I then gift the excess grains to people who have asked me for some... building up a network of *backup* grains in the event I lose mine somehow.
I have drank the raw milk\kefir as much as 6 weeks old without any issues or problems. I use raw goat milk exclusively.
If I chose cow milk, I would ensure the milk was from old world cows, such as Jerseys, Asian or African cows (not certain of breed names). Guernseys are said to be okay too. These are generally light to a medium brown cow... not the black and white, such as the Holsteins.
Also pastured and primarily grass fed (some will use organic silage and grains in the winter, which is okay, but can change the nature of the milk), is the very best.
Goat milk is even better IMO... as the proteins are more available to humans, the fat is naturally homogenized (it is very short to medium chain), and it digests in about a third of the time a cow milk will.
If you only have the option for a pasteurized, homogenized, Holstein cow milk, I would go with the no fat as the fat becomes indigestible when it is *artificially* homogenized. This is what causes most people digestive problems with processed milks.
In a pasteurized milk, the kefir grains *should* greatly improve the milk's digestibility and nutritional availability considerably. Pasteurization renders most milk nutrients, including calcium, void or unavailable due to chemical bond changes.
That said, I have absolutely no idea how the grains will do in a skim or lowfat milk... I have only ever used a whole, raw milk.