Wow, thats a bunch of questions.
refugium :lol:
Caulerpa, there should be some one local with some, but if not it can be posted to you.
The only miracle is that people still buy it, apparently it's been analyzed and it is just garden dirt, even had decaying plant matter in it.
So long as it is clean just put it in the sump, it will get seeded from your other rock. It would only cause changes to your water if it has lots of 'stuff' on it that starts breaking down. If it is dirty do a google search on "cooking live rock" basically involves putting it in a dark barrel of salt water and letting it stu for a few months.
The trickle filter is at least one of the causes of your nitrate problems, best thing to do is remove it slowly, maybe 1/4 per month to let the balance of the tank adjust. I guessing you have sand in the tank as well, more than 1cm and less than 15cm? If so its highly likely this is full of rotting detritus.
Have a look at this.
http://www.fnzas.org.nz/fishroom/viewtopic.php?f=8&t=2406