been using the 600mm f4.5FD... I've also noticed this video stuff is HARD
These two statements are related... What are you filming with a 600mm lens on an aps-c camera (effectively making a 960mm lens)? Birds?
85mm on aps-c is close to 125mm FF - which is a fairly standard length for close ups. I've been up to 200mm when filming events from the back of a hall, but at 600 you'll need static shots and to be somewhere around f16-22 to get your subject in focus... At 600mm on a 60D at f16, if your subject (a bird maybe?) is 50 feet away from you, depth of field is just over a foot. And given how quickly birds move, maintaining focus will be extremely hard. If the bird is only 20 feet away your down to about 2 and a half inches in focus...
If you're new to video I'd suggest trying some more reasonable focal lengths and subjects first... Wildlife videography is about a tough as it gets - tracking big cats making a kill is about as far away from a controlled studio setup where you can have multiple takes as possible. And this is a place where having shallow depth of field from a DSLR(as opposed to a videocamera with a 1/3 inch sensor) only makes life harder.