I agree with Silvani ~ If you're going to give the Mollies salt, I wouldn't have Otos. The Guppies and Platies will adapt to the salt just fine. If you have hard, or alkaline water, salt isn't such a big deal, but it's usually safer to add it, even if it's not neccessary. (I know everyone will come, and say they haven't kept Mollies in salt, and they were fine, etc, etc, but I have always had issues with freshwater tanks and Mollies. I have hardish water, and it's not my fishkeeping skills, since I have kept more fragile fish.
) So it's my opinion.
I wouldn't mix the water. The water shouldn't be that different, if it's coming from the same tap, and you're doing the same thing to it. Just make a quick check before you move her, and adjust accordingly. However...if you're going to add salt to that tank, then you have a bigger issue. You'll have to go into acclimating them.
For the gravid Swordtails ~ Wait unil she is pretty pregnant, as nets can stress them out greatly, and you don't want aborted fry, or birthing problems. I would personally recommend that you just put a lot of floating plants in the tank, and let the fry survive that way. Or possibly catch them once they are born, and put them in the trap.
Sorry, I feel like I'm shooting down all your ideas.
You really have a good plan, I'm just nipping possible problems in the bud.