DD1 has been doing regular ballet for 3 1/2 years. Two recitals a year, and a solo in the spring show. The school she attends is attached to the gym and makes accomodations for gymnasts. Classes are always scheduled on off gym days. Our gym does not offer ballet or any dance especially for gymnasts, although they talk about adding dance every year.
I have found it to be good for her, more friends and something else besides gym. Gymnasts who do dance seem to be a little more prepared for the dance part of their routines on floor and beam. But it has to work for your family and your schedule. And if you think leo's are expensive, wait until you have to buy pointe shoes, and you even have to sew the ribbons on when you buy them! One of the other families at our gym tried to do ballet at a different studio and the schedule was a real headache.
All that being said I recomend dance, if you can fit it in, it's a natural for our little performers.