Richard. The 3.0 version of Media Composer will let you play Progressive and Interlaced clips on the same timeline if they are the same frame rate. 3.0 Also supports 35 Mbits XDCAM HD EX. Version 2.8 only supported 25 Mbits thin raster 1440x 1080. You can edit native in the XDCAM EX 35 or 25 Mbits format in 3.0, but will need to render your effects in an Avid format. Also, when it is time to output to tape you will need transcode to an Avid resolution.
Are you using the Sony EX1? Is 1080 30P an option or are you doing 24P with a 3:2 pulldown?
Let me know.
Avid ACSR