SSD instead of DVD temperature
Since the SSD is likely to be powered on much more than the DVD ever was, your thermal load inside the mac will be higher. Most SSD use even more power at idle than HDD, but it won't be nearly as bad as the CPU/GPU running full tilt. The fans may run faster to compensate if the ambient temperature is high, but I've heard no hints of problems even from people in warm environments with SSD+HDD.

There are many apps to log the temperatures so you can get a before / after log and check on things as you should whenever you add parts not planned by the manufacturer.