You seldom see more than 6x Freighters on a dyna mission, and then there are usually 2x draft spots on either side. That means you have 10 models + all the PF's and ftrs that may also be present.
I'd ask, in the interests of seeing these freighters be used on a dyna, that you keep the polys under 7k.
Here's another thing to consider... Freighters are really good for only one thing. And that's to blow up! So make the break mod real pretty if the dyna is your target!