ATA connection flights land at the far end of the airport and then bus you to a gate at the end of the ATA terminal. However, I believe the time is built in to the schedule. It is probably a good 10 minute walk from one end of the A terminal to the other. then you will hit the intersection of the other terminals. Southwest has all put the first couple of gates in B. If you're in one of the first southwest gates, you'll be there in a minute or so. If its one of the further out ones, maybe 10-15 minutes.

If you haven't checked in, there have been Southwest Check in Kiosks in the gate area. (I'm not sure if they are still there)

Southwest is usually pretty easy-going with its 'flat tire' rule. If you're destination is some place like MCI with flights every 30 minutes to one hour, I'd go for about a 30 minute connection. It should be physically possible, and there is ample fallback. If the ATA flight is later in the day, or your destination has more infrequent flights, I'd schedule at least 60+ minutes just to be safe.