Simple. Let AE do the work.
Your footage does not have the same frame dimensions but it does have the same square pixel equivalent.
Your DVC Pro HD footage is 1440 X 1080 with a 1.5 PAR, the XDCam is 1920 X 1080 with square pixels.
All you have to do is set up a standard HD composition with square pixels, then drop your DVC Pro HD footage in that comp and AE will make the conversion. It's always best to work in square pixels anyway.
The simple rule for working with non-square pixel footage is to interpret it as it is, not what you want it to be. After Effects usually interprets footage correctly but it can make huge mistakes with still images and with DV and Widescreen DV footage. If an image or footage is a standard non pixel dimension (720 X 480 for example) AE is going to assume that footage is non square pixels. In this case it could either be 4:3 or 16:9 footage. AE would guess that it was 4:3 so if it's widescreen you'll have to fix that. There's not as much ambiguity with HD footage but you'll still have to watch out for still images and graphics created in Photoshop or Illustrator. Make sure you know what the footage is supposed to be and check the interpretation in the Project window thumbnail to make sure it's right. Then happily and easily mix footage from any source with footage from any other source in a square pixel comp and you'll be just fine.
thanks rick for that detail explanation, as I started with the editing/compositing and was going crazy trying to understand why footage with different FRAME SIZES fit into the same comp/sequence without needing scaling.