They can be used together, but one problem with the creatine and ECA mix: creatine can cause a problem if you don't drink plenty of extra water while taking it. Caffeine is a diuretic and makes you lose water. Stimulants in general can cause adrenal gland hyperfunction, which can also cause fluid hydration irregularities. Combining these two supplements can lead to hydration problems if you aren't careful. Regardless of whether you are using creatine or ECA, if you experience muscle cramps or have any symptoms of dehydration, I would stop using them immediately.