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.
personally I always looked at group fitness as an add-on to my personal training. There is value of being exposed to a large group of people. I have never seen a group fitness instructor, though, who was able to make REAL money with group exercise even if many classes a day are taught (which also increases the risk of injury exponentially). I only teach MELT classes which is a as yet rather little known specialty.
What I observe with my group ex colleagues, though, that their value goes up when they are able to teach many different modalities, in fact, it is a pre-requisite to being hired. As Ariadne pointed out, all those extra certifications cost a lot of money, and I sometimes wonder whether some instructors really make a cost/benefit analysis. Mind you, the fun of teaching a class is a factor without monetary value but of great importance.
I assume that you already love Pilates and are drawn to it from that perspective. If the market will bear it, it is still a specialty worth looking at. I know that Pilates instructors in some places are not getting any more compensation than other instructors.