that's fair. i wanted the wallpapers/ringtones/books. so that's what i got.
as for "messing up" your tier, no. it won't. You'll just go to the "manage my pledge" section (from the button) and then you can select a new tier. In my case, i started at 20, then bumped to 50, then i added 7 for the guide. you'd just do the same thing. Manage, then select the tier you want. it'll change your pledge to that tier amount, then you can manually change the pledge to put more money in, for any of the add-ons.
hope this helps
Just remember to up both your pledge and your desired reward lev