ActionScript 3.0 :: Targeting Nested Clip From Class?

I use the loader class to load in an external swf, a preloader movie in my class. (not a document class)

To target the properties in the swf (specifically the text field in it), I coerce it to a movie clip after it's loaded and add the child with:

var thumbPreloaderMC:MovieClip=evt.target.content as MovieClip;

now I can use:
thumbPreloaderMC.preload_text.text "sometext";

to reach the text field. works.

But, I want to nest that clip in a container movieclip I'm placing all the thumbnails I'm loading in, thumbContainer.

So I create the movieclip thumbContainer, add the child, then add the preloader swf to this clip.

I can't name the loaded swf, it throws an error, I googled that and see I can't name an already named clip.

I get an error "TypeError: Error #1010: A term is undefined and has no properties."

First of all, I'm fairly green with as3 still, I think perhaps I should rethink how I'm structuring this. I could and would ask how to target a nested loaded swf from a class.

is nesting clips like this from a class bad practice? What are best practices when it comes to using timeline based animations like preloaders etc by a class? I'd like to make my class as reusable as possible, but one can't create all animations and graphics from code.