I use "normal" fuel tubing for the clunk lines on all my tanks with no problems. You don't need extra-flexible tubing at all. It's true that if you are very nose-down for a long enough time, you can get the engine to die, but usually the engine gets stopped by something else (the ground) before it becomes a problem. Actually, because of the forces on the plane and the fuel acutally in the lines, you'll find it really hard to actually get the engine to even sputter in normal flying and even with "extended" downlines.

As for the flaps, they won't hurt anything, but on that plane, you won't really need them either. If it was me, I'd set them up if you can afford the extra servo and have a channel for them, but don't even think about using them until you're well past soloing. Then you can take it up high, hit the flaps, and see what they do. A guy at my field has a setup simular to what you describe. He can take off from extremly high grass/rough ground, it almost looks like the wheels don't even turn on the ground. And he can land almost vertically with out much of a breeze. Kinda cool for goofing around. Anyway, if you don't have hte servo and spare channel, then just fix them in the up possition and don't sweat them.