I am 17 in 2 months and have not yet finished breast development, Ifirst had my period 6 yrs ago. What would be the effects of taking soy isoflavones?

Not finished ??? Hate to bring unwanted news but you are likely done. Breast development is based on a complex interaction between genetics/ tissue receptors and hormones. The outcome varies between a raisin on a pancake flat to a watermelon. Isoflavenoids would not change your status.
None. Taking soy isoflavones with not do any harm, nor will it do any good. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Drink enough water daily so that your urine is mostly colorless. Practice safe sex. Get HPV vaccine.