Sounds like you have several good spots. Don't get all hung up on details. What type of tree it was when living has little to do with whether it is usable once dead and totally dry. Try to keep an open mind when looking for wood. It is real easy to look at a great piece and think it is too big or has some defect. If you have a saw, many of those defects can be sawed off or stuck in the substrate. The saw you carry is to get it down to a size to carry but when you get it home, you can reshape a lot of things. Don't worry about color too much either. Dry wood will change color once it soaks in the tank.
I do an overnight bleach soak just to clear any potential pests or pollution. A simple rinse to dilute the bleach and then an air dry will give you good insurance. Boiling a piece that may be three feet long just isn't practical and does nothing for any oil that it might be holding.
Thanks for the tips Me and the girls might take a walk this afternoon and see what we can find along the dirt road we have a lot of down trees from a storm about 6 years a go. So may be we will find a great piece we can trim to fit in our tank we are hoping to make a tree. I hear root work well for this. We will see.