Another thing to consider is how the owner of the rights to the clips would feel about you using their material to raise money for your film. If it were some corporate newscast or promo film of some mega-bucks rock band, I'd say don't worry about it. <puts on flame-retardant suit> On the other hand, friends of mineâ€”starving documentary filmmakers who risked life and limb to get dramatic footageâ€”have had people cherry-pick their films to make fundraising trailers for their own films, sort of conveniently forgetting to mention to funders that they haven't shot an inch of tape themselves. It really sucks to find out that others are using your work for their fundraising while you yourself are living below the poverty line.
The point is that even if you intend to license the footage at market rates later, if you're using footage owned by a real person who is not a multi-millionaire or faceless corporation, you should have some arrangement with them. Yes, you can probably get away with it, but it's not ethical.