Dave, if you're doing well with the casei, go ahead and stick with it (it is SCD legal). Lots of people use it! You could add acidophilus if you want, but it's not mandatory.
GI Pro Health carries acidophilus (called SCDophilus). I believe Custom Probiotics (online store) also carries it. I'd ask at the BTVC board about
adding it to your starter, because I'm not sure how much you would want to use. Oh, and your could use a little Dannon of course. Make sure it's full fat, because the others have pectin in them.
I'm thinking about
expanding to kefir. It's considered super advanced, but maybe I'm ready
.