We are looking for a small group (12 - 16) tour company to Croatia. We have used OAT for several (6) tours and the trips and guides are generally quite good--the customer service department is positively horrible.
Any experience with other companies would be appreciated.

My favorite tour company is Rick Steves. I have been on four of his tours. I have been on two Gate1 tours, but do not like those nearly as much as the Rick Steves tours.

I was looking at small group tours and noticed one of the companies I was looking at has some Croatia tours. I haven't used them and I know nothing about this company but they offer what you're looking for. Do your due diligence.

http://www.backroadstouring.co.uk/croatia

Sights and Soul Tours does a women only tour to Croatia. They are a very good tour company with excellent hotels, and high quality restaurants for included meals, and have excellent guides. The group is limited to 16.

The Gate 1 tours to Croatia are run by Kompas a Slovenia based tour operator. You can see the offerings on Kompas.net It is a pretty good tour for the price, and it is a reputable company but I don't know that it would be limited in size to just 16 people.

If you are traveling from the USA there are no non-stops flights to Croatia so if you are not using air miles, you may find a travel specialist who can get you a better deal on a flight

Road Scholars has one.