I found a beautiful pattern for a baby girl's sweater that requires Debbie Bliss baby cashmerino. I don't currently have that yarn in my house, but I do have Plymouth yarn baby alpaca.Baby alpaca is a dk weight and baby cashmerino is between a dk and fingering weight. Can I switch the two?

There's only one way to tell.
Make a good-sized swatch with the recommended needles and see if you get the gauge in the pattern.
If you get too few stitches per inch try again with smaller needles.
When you get the right gauge, check it carefully to see if you like how the fabric looks and feels. If you do, you're home free.

Two things about alpaca: it's very warm and it may have a tendency to stretch. Shouldn't be a problem for a baby sweater.