[QUOTE=GallopGirl;8058961]
I’m a hard one to fit. I am 5’10 and 130 lbs. I wear a 4/6 in normal clothes but have to buy a 12 long to 16 long in hunt coats because my size just doesn’t fit in the shoulders and I can’t lift my arms because the material is too restricting. It’s a pretty common issue to have any type of clothes not fit right off the rack. Find a great tailor, fit the largest part of your body, and the tailor can take in the rest to make it look amazing. He literally takes a foot of material out of the mid-sections of my coat. They look great![/QUOTE]
Not quite the largest part of your body, but rather with a tailored coat of the type riding coat are, you REALLY if at all possible want it to fit your shoulders well. The shoulder area of a tailored coat is pretty complicated, so alterations there can be tricky, and time consuming, and occasionally downright impossible.
However, with modern garment production techniques, there is often also not a lot of spare fabric left in seam allowances to let things out - so if you have a choice between a perfect fit in the shoulders but tight in the bust, and a touch big in the shoulders but a good fit in the bust, you’re likely to be unhappy if you go for tight in the bust and assume it can be let out successfully. A good tailor may be able to make a small adjustment in the shoulders like adding a tiny bit more padding so it no longer looks too big. Depends a lot on the construction of the garment.
(Of course, your best bet if you are in a situation like that would be to get both sizes from a place where they can be returned, and then go and actually consult with a good tailor in person with both coats. That way they can see how each fits and how each is constructed and how the fabric behaves - some fabric really holds on to seam lines like where the seam allowance is folded back and pressed, so if you let it out you can tell because there are these marks where the seam used to be - and give you advice about which they feel will work out best for you. Then you have the one altered and return the other and there you go. 
But yes, something like nipping in the waist a bit on a coat that fits well through the shoulders is a pretty simple alteration and is done all the time on men’s suits, so most competent tailors should be able to do it with very little trouble which means it shouldn’t be terribly costly, either. The more complicated an alteration, the more time and knowledge is required, which means the more it’s gonna cost you. 