ive read some articles and post on balancing and trimming aerobatic and 3d planes. but it seems that most are for non epp planes. you guys that have the flat epp's how to you go about balance'n and trim'n your plane at the field.
Here's my opinion: It's great that you're reading all this stuff, because it makes you understand how things work and why you want to trim and mix. In fact, I like your style a lot. You don't take anything for granted, but you ask questions and try to build up your knowledge piece by piece.
That being said, when it comes to your flat EPP foamies, I think it's more the spirit of those articles you want to have in mind, rather than the exact procedures. Flat EPP foamies are pretty inexact in their design and their flying characteristics. Also, people tend to put pretty cheap servos in their foamies, and those servos may not center perfectly, so chances are that your trim isn't going to be worth that much anyway. Make sure your trim is decent and your CG is about where it's supposed to be and then go fly the snot out of it. It's OK to experiment with the CG to e.g. see what it feels like to fly a tail heavy plane. You don't want to do that with your 50cc balsa model...
And by all means, if you want to experiment with a KE mix on your foamie, it's probably the best place to do it, since whatever happens, you probably won't cause any lasting damage to the model.
I guess what I'm trying to say here is to relax and have fun with your foamie, but that you're doing the right thing by trying to learn as much as possible.
Study, fly, experiment, fly again. Rinse and repeat!