Checking to see if it's possible to mix video shot in 1080i with Standard Definition video in FCP 6. Would this work and allow me to have video look alright? If not, what changes would I need to do in order to make the work. Please advise. Thank you.
Not usually a problem, but you may have to do a lot of rendering in the timeline, depending on your system.
Most will give you the green line (which means it will play at reduced quality until render). If your system is marginal, you might not be able to get any playback without render.
You could convert it all to your SD codec in Compressor first, but it may not be necessary. You should test it out. If after doing a render, you are unhappy with FCP's downscaling, you may want to use Compressor with the Frame Controls on, set at Better or Best for the resize.