You need to take a look at all your various media assets ( movie clips only) and get a feel for what the codecs are and the various resolutions. ( pixel x pixel dimensions)
eg 1080p, 720p, DV etc...
I would advise creating a sequence that matches the lowest resolution asset.
That means you scale your highest rez assets down to fit. ( easy and can be automated).
Make a little list of what you have got , post it here and some one will advise.
What was the profesionally digitised media done as? ( ie file type , size)