More or less, yes. CAF's website say's Houston's model is the "Mc-T-Mc" configuration, while the Purple Line model will be the "Mc-R-S-M-Mc" configuration. The letter combos relate to the tram modules, so it looks to me like the end cap modules will be the same, but the central modules will be different somehow (in addition to there being 2 more of them). I can't find a legend to tell me what those letters actually mean beyond that, unfortunately.
Regardless, CAF will make anything from a 3-segment version up to a 9-segment version. Check out Budapest: 183 feet long (in the "Mc-S-T-S-M-S-T-S-Mc" configuration).
gabor timar on flickr