I thought only exclusives can use FTP? I upload via the online page, just like with photos. It takes a long time and sometimes goes wrong. They take a very long time to review, it can take over a month.
I would recommend uploading one first, to make sure it is accepted. Having a large batch of videos rejected with istock would be a nightmare. They are quite lenient but I have had some rejected that were accepted on 4 other sites.