Do your SD tapes contain 24p material (i.e. with pulldown?), if so, capture in a NTSC 23.976 project. One captured, switch project to 1080p/23.976. SD will be scaled to fit 1080 via an uprez.
You cannot capture DnxHD36 via FireWire, this is only done via HD-SDI and while in a 1080p 23.976, 24, 0r 25 project type. You will need a Mojo DX at the very least.
Mojo DX is digital only, and while it has an SD downconvert function, the I/O is only HDMI or SDI. Analog component would be done via an external Digital to Analog converter from someone like AJA or Blackmagic.
Michael
Michael Phillips