Yes, on DPP-4. The standard dose is 5 mg for saxagliptin and 100 mg for sitagliptin for inhibition of the enzyme dpp-4. The two molecules are very different and might have different effects. Sitagliptin has many fluorides and could interfere with iodine metabolism in a situation of iodine deficiency, for instance.
Yes. Although, these are two different chemicals, they belong to the same class of anti-diabetic medications. Roughly, the doses presented are equivalent and can be interchanged.