Cyclone V IO Drive Power Maximum Capacitive Load

100MHz is fast for external I/O. With an external interface you have not only the internal capacitance to deal with (including your I/O protection and connector), but also the external connector and cable or whatever is out there. 5-15pF may be no problem depending on what you're doing, but I would recommend speaking about your application with the vendor of your TVS array. In these days of on-line forums like this one people seem to turn to vendors for information much less frequently. They have resources to help you so go ahead and use them.