Your larger city services, i.e. Dallas, Ft. Worth, will send you throught their school. Smaller towns will accept a basic certification from the Texas Commision on Fire Protection, but these positions are highly competitive, most averaging 40-50 applicants per open position. Texas A-M fire school offers its 12 week program several times per year, but it is a dedicated 12 weeks, no work, no outside schooling.
You also have to ask yourself, do I want to be a fireman??? Most people do not think about it but there is a reason that many fire department based EMS services in Texas require fire certification. They want you dual trained. It is a wonderful career path to pursue, however it seems to conflict with you previous posts of wanting your RN. If you get involved in the fire service, they will want, if not demand, longevity. They are not going to spend thousands of dollars to train you if you are going to leave within 3-5 years. Some fire departments will have you complete a contract for your training, quit before their terms and you could be paying them back for training!!!
From what I've read from your posts, you seem to have a good plan; continue your education and start employment with ETMC when you are of age. Remember, your first instinct may be your best.........................Take care................