Workflow for HDV to SD delivery

Sony = PAL HDV @ 50i
Canon = PAL @ 25p
GoPros = PAL @ 25p

All sound about right?

Because going from HD to SD isn't an even conversion, you'll need to deinterlace the 50i footage before resizing to SD.

I would simply use an HD timeline at 25p, deinterlace the Sony footage and then downconvert to standard definition on export, and let that conversion handle the re-interlacing.

I don't understand what you mean when you say you want to avoid DV. Is the broadcaster looking for delivery on DV tape and you don't want to use a DV tape deck?

You'll need to adhere to the delivery specifications of the broadcaster/network. If you know they run on Avid, offer to deliver with an Avid encoded file, you can get the codecs here and they are compatible with Premiere when exporting to MOV http://avid.force.com/pkb/articles/en_US/Compatibility/en372311

Second question can I edit with the gopro footage and deinterlace in premiere timeline or should I convert footage beforehand?

GoPro footage is h.264 and, for all intents and purposes, should perform in a similar manner to DSLR footage.

In terms of deinterlacing, once you have your segments cut, right click them in the timeline and select Field Order... and you can select "Always deinterlace"

Premiere discards one field of each interlaced frame, and then fills in the gap. You'll lose a little quality in terms of horizontal resolution, but you're also downconverting to SD, so I don't think you'll really notice any real hit to quality by the time you complete processing.

Alternately, you can deinterlace in After Effects with some more robust plugins like Fieldskit or Magic Bullet Frames. Some PC users also use something like AviSynth or VirtualDub to process deinterlacing as an intermediate step. Just food for thought, but I don't think you have to follow this more advanced path for your situation.