You can use anything for a starter that has a certain level of *live* (yogurt making**) bacteria in it.
That usually means either a powdered starter (like Yogourmet's, which I use btw, which would be one packet per quart/liter of milk) or a sufficient amount of plain yogurt that you've purchased (or even a flavored yogurt, but then you're committed to that flavor, etc.)...or a yogurt you've made (not too old, or even frozen for the purpose).
There are other brands of powdered yogurt though besides Yogourmet, but they all tend to taste a little different... they're convenient though, and when using powdered starters you never run the risk of having had other bacteria introduced into the yogurt you'll use as a starter after *it* was opened--probably not a big deal if it stays covered, you don't stick your fingers or other "dirty" items into it, etc.
Another good reason to use a powdered starter or a specific powdered starter is that the amount will always be sufficient and the resulting yogurt should always taste exactly the same (if you've used the same milk/etc), compared to using various store-bought yogurts.
As for "probiotics" changing the instructions, since the starter bacteria are themselves probiotics (and the resulting milk/yogurt contains a lot of probiotic beasties), adding more probiotics in the form of a pill or something would probably just mean the yogurt would take less time to incubate to a desired level than it would have with powdered starter or more (sufficiently active) yogurt. There's really no need to add additional "probiotics" unless you want to add a particular strain, etc, and the benefits of the basic-2 are great already (gut health--including regularity, helping with various bowel diseases*** and difficulties, etc.... as well as immune system help, etc.)
**by law in the U.S. that has to be these two strains --lactobacillus bulgaricus and streptococcus thermophilus (…lactobacillus acidophilus is optional)-- but yogurts can always add other strains if they want too since there are loads of them and many that are similar to each other but can be tweaked a bit
...anytime you buy yogurt for its probiotic effects (eating or for fermenting milk), be sure the container says "contains" live cultures/bacteria, not "made with" live cultures--since they could actually have been killed by the next step in processing done by some manufacturers
***if you have Crohn's Disease or colitis or something really serious (not just IBS, constipation, etc.), the best results from yogurt have been gained by removing *all* the lactose (milk sugar) from the milk/yogurt which usually means incubating it for a full 24 hrs at about 110-120 F... that will make it pretty sour though
***if you're lactose intolerant, removing even a little of the lactose (like what you'll find in store-bought) is usually enough for most lactose-intolerant people (you can always add sucrose/table sugar back after incubating if you want, which is what most people do anyway)
I've written a bunch of stuff on the differences between store-bought yogurts and homemade, various ways to make them, stuff to add as flavorings and ways to use them, etc., if you're interested in these previous questions:
https://answersrip.com/question/index?qid=20070405080017AAQBRCv
HTH,
Diane B.
ADDED LATER:
I just looked at this particular yogurt maker, and it's like most other electric yogurt makers except that it makes a large quantity (2 qts) if you want. It also uses water to create a "heat bath" which supposedly keeps it at the right temp (but all yogurt makers would do that).
This sentence bothered me though:
"Also it is the only yogurt maker that will make our exclusive Yogourmet CBA yogurt that contains L. Casei which can boost your immune system, and B. Bifidus which can help promote regularity."
I can't imagine what this means unless their particular added strains are slightly more temperature sensitive (to being killed) than the regular strains, so the water-bath type supposedly holds even the bottom of the container at that slightly lower temp.
I saw this which seems to support that idea:
"And caution if you use the CBA starter, do not put the water in the basin with it plugged in until you are ready to put the yogurt in, the water will heat too fast. This culture needs to gradually reach a little over a 100 degrees and not go into it right away."
And that stuff about the "special" strains found "only in their yogurts," that could technically be true but only because as I mentioned before there are loads of strains and anyone can tweak a strain to be a *bit* different and use that as a marketing tool (and name it themselves).
In fact *all* yogurts "promote regularity" and "boost the immune system." (Activia yogurt pulls this same nonsense in their marketing commercials, but probably makes them more money because most people know little about yogurt itself.)