First off, welcome to geocaching, and to the wonderful world of coins and travel bugs!
Then for the answer to your questions:
You are perfectly allowed to pick up that coin from the cache. Just be sure to log it, and to move it to a new cache in a reasonable amount of time (less than 2 weeks is good), and make sure you log it into that cache too.
'Trade rules' (trade up, trade even, or don't trade at all) are a little relaxed around travelers. Since they like to move, feel free to help them on their missions, even if you might not have a trade with you right now. It is nice to swap if you can. Or if you want to go back and put a replacement in that cache that is fine, or put something extra in the next one. It all works out in a global way.
Your TB idea sounds very nice. I have seen TBs on everything from toys to books to keys to stuffed animals to pictures to .... and I enjoy reading about and moving every one. The only caution is that something nice may disappear from circulation.