Can I add Видео which were made by somebody else!For example I downloaded video from Youtube, will it be legal to add it here?

well its okay because Fanpop doesnt actually HOST the Видео they are just linked. Ты dont download it to your computer and Загрузить it here (which would be sketchy if Ты didnt make it).
Ты post them as a link and any video can be traced to the original hosting site (youtube for example) by clicking the little blue Стрела Далее to the Название (see image).
thats why- if i really like something- i always click on that link and Комментарий and rate the original as well as the Fanpop content but since not every one does that i think its good to credit the video maker.
p.s. if the creator doesnt want their Видео to be Опубликовано elsewhere they will disable embedding so Ты can only post it as a link and not a video thus bringing the viewer to the original content.

yes Ты can, but Ты have to mention who made the video, или else everybody thinks Ты made it ;)