matching non HDV footage with HDV on FCP

I'm working on editing with HDV footage on FCP 7.
I tried to edit with various old footage.
When I converted old ones to HDV format using MPEG, clips were stretched.
When I didn't change frame size and format and drag it to timeline,
I need to render every time.
I wonder how people mix various format and frame sized clips on HDV form.
Look forward to listening to good solutions. Thanks.