It will be fine so long as you dont exceed the height limit which is marked on the seat post. If you need the seat all the way forward then a zero set back post would be better. They may have said it is not recommended because it may be better for you to go to the next bigger frame but this will also increase reach so may not be good for you. Just put the seat where you wnt itand make sure you keep an eye on the limit on the seat post

Both alloy and carbon posts are designed to be strong enough