There are two potential problems I see, neither of which should prevent you from using a Decathlon for most of your primary training.
First, you will need to have at least 3 hrs of flight by reference to instruments and demonstrate this skill on your check ride (FAR 61.109 (3)). Many Decathlons don’t have any gyros, not to mention a full ‘six pack’ on the panel. Second, some are not equipped with any navigation radios (probably not an issue if you are using an ACA built aircraft). Use of electronic navigation methods is another skill you will need to show the examiner.
You could do the few hours of IFR work with just needle/ball/airspeed in a Decathlon, but it would probably be more productive to use a different airplane for a few hours to learn basic instrument skills.
My Super D was used by the previous owner to teach his son to fly and he loved the experience. If you’ve got a good instructor with lots of time teaching in tailwheels and insurance which covers solo student pilots, consider yourself very lucky. Go for it!
Paul