Is it safe to take modafinil as a study aid while on a ssri?

Yes. Yes. Modafinil is safe to take with ssri's. In fact, many psychiatrists use Modafinil as an adjunct to ssri's in treatment resistant depression. I have had a lot of success prescribing it to patients on ssris who have persisting energy and concentration deficits. Just make sure that all your doctors are aware of every medication you're taking.
Prescription? Because you are using the phrase "study aid" -- is this medication a prescription that you have been given by a doctor? A doctor can evaluate the side effects and drug interactions of medications and discuss these with you. There is no safe medication -- not even over the counter medications. And those that enhance attention should only be used when prescribed by a doctor.