Yes, the powdered form of Milk Thistle is fine. The liquid forms are simply the powdered forms compounded into an oral-suspension. Just make sure that if you are using the powder from any Milk Thistle "capsules" instead of just straight Milk Thistle powder that there aren't any "fillers" inside of the capsules; most will contain gelatin, which is fine, but some of them put other fillers inside that aren't fantastic....
I find that powdered-supplements are much easier to give animals in-general than liquid ones, because you can easily mix it into their food, like seed-mixes or chop recipes...For example, I give my birds their powdered Probiotics mixed into their daily portion of seed-mix, and I give my Beardie her daily Probiotics by crushing a tablet and adding the powder to the Calcium and Multi-Vitamin powder that I dust her live insects in.