ACSM is a great certification to have, and with a basic group ex. you have a lot of options on what you want to do.
Layering a specialty on top of group ex can be a good thing. Do keep in mind that every one you add means added cost in training, certification, CECs, etc… so I’ve always thought it is a good idea to think carefully before deciding. There are several factors to consider: relative wages (there is data on wages for group and specialty exercises, such as pilates broken down by area of the country available on this site), marketability and saturation in your area (I do not know Dallas, but it has good population density, and I think there is a good slice of people able to afford fitness, but that would be useful to look at), and personal interests.
I have done Pilates myself, and mix it in with my other classes often. I have not chosen to get a real Pilates certification as I am very committed to my yoga, and there is significant cost and time required for Pilates. As I am not able to be full time right now it does not work for me. However, I know quite a number of people who have done very well with it. If you find a local place that certifies in Pilates you might also ask them if they help with placements once you finish.
I think the expense and the time required though, if you are going to make a long term and significant time commitment to the discipline, are well worth it. The same thing that makes taking the training difficult for some to do is the same thing that makes it worth while. With ACSM and PMI you would certainly stand out as someone who cares about being informed and well trained.