Yes, Sandor recommends that as well, and I added freshly harvested huckleberries which were perfect.
:-)
I am on an island in the Puget Sound.

You are partially correct. Not all yeasts (in this case, wild yeast) are alike and you should know about the kinds you might be cultivating. Also that the wild yeast from the central west coast is prized for the flavor and texture it imparts to bread. Usually referred to as San Francisco sourdough the yeasts are either or both of Candida milleri or Saccharomyces exiguus along with the bacteria Lactobacillus sanfranciscensis types 1 and 2. Actually there are symbiotic groups of yeast and bacteria, together referred to as wild yeast, in nearly every part of the world. Some are remarkable enough for names, Tasmanian Devil, Red Sea, Yukon Gold. Others are less memorable, but each has a flavor unique to the geographical location it comes from.

While perhaps a bit tactless, chopper is correct. You shouldn't add milk to the starter. Use it in the bread recipe, absolutely, but not in the starter. Once your starter is established, you should only feed it with flour and water. Some people insist on adding fruit and milk or yogurt to help establish the starter, but the yeasts and bacteria aren't necessarily compatible. Sometimes people get lucky, but the wrong kind of bacteria will conflict with the yeast.

The bacteria break down starches into sugars, which the yeast feed on. The yeast produces byproducts that the bacteria feed on and the bacteria produce chemicals that inhibit the growth of fungus (natures own preservatives). It's these chemicals that give sourdough its flavor and texture. These symbiotic relationships have evolved over millions of years. Better to let them pick their own playmates instead of trying to force them to work with strangers.

If your starter is bubbling happily away but the bread isn't rising enough, temperature is the probable culprit. Wild yeasts have personalities. It can be fussy, temperamental and sometimes uncooperative. Easiest solution (you're probably not going to like) is add baker's yeast into the bread mix along with the sourdough. The wild yeast will impart flavor and texture. The baker's yeast just makes bubbles. Baker's yeast was developed so that bread could be produced on assembly line. It has no companion bacteria. It has a mild,(some (like me) would say bland) flavor, but what it does, it does well and consistently.

I'm not an expert, but I've picked up a few things over the years. I've just recently gotten back into baking sourdough. The starter I had in the refrigerator was unrecoverable. I had managed to revive it before even when it had been inactive for almost a year. But I had ignored it for longer this time and nothing I could do would bring it back to life. I started a new batch and have been very pleases with the results (and have put on a couple pounds in the process).

Baking sourdough bread has at least one thing in common with playing chess. It takes about an hour to learn and a lifetime to master.