Of course it is legit. In mixed doubles, men will often make more errors off the lower-level women's serves than the higher-level men's serves. They just aren't used to them. Some guys don't like to generate their own pace and struggle more off of slow balls.
Of course, if you used it all the time, your opponents would catch on and start beating you, but mixing in variety (as you did with putting in a few fast serves) is almost always a smart play. Find out what your specific opponent does or does not like.
You have learned an important lesson.