
Hey, Boss (ironically, my real boss would not appreciate the amount of time I'm going to take to answer your questions!

) Woo, it's Friday! OK, here goes:
In your experiences what was the easiest type of extension to apply yourself? I would say, either clip-ins (made myself) or tape/glue extensions.
For thin/fine hair what type of extension shows through the least? Hmm...if you don't go up too high --- clip-ins are pretty undetectable. If you want to go up high, I would say, either single-weft tape extensions or skin weft or hand-tied extensions.
Has anyone used skin weft extensions, and how did you feel about those? I'm currently using a mix of three methods, including skin wefts up top (close to part). I LOVE them! They are pretty undetectable, flat and comfortable. Easy to install & remove. And you can reuse them. The only weird thing about them is the buckling that can happen if you don't cut them or apply them right. They can 'bunch' if you don't cut them into smaller pieces.
My hair is already damaged, what type of extension is the least damaging? As many will state here, most extensions methods can be safe is applied correctly and removed correctly. I can't fairly judge methods like fusion or microlinks since I've never worn them. In my experience with clip-ins and tape methods; they are non-damaging, even with a heavy hand. I workout every day (except Sunday) and deep condition with them in (the tape that is) and I have had no problems with them holding. my hair is growing and getting healthier by the month!
I want long term extensions. What extensions would you say last the longest? Oh boy...ummm....either sew-ins or fusion. Others can probably help more on this one!
What extensions would you say tangle the least? Geez, I can't fairly say. I don't know if it has so much to do with the method as it does with the type, texture and quality of the hair you install.
Can you still put your hair in a ponytail if you have extensions? Yes! Now, I don't do high ponytails, but I can do a nice mid-level pony with no problems and the extensions are hidden. Even with clip-ins I can do ponies.
How much hair do you need to buy? for a full head, I would say about 8 oz, just to be safe. You may not need all of that.
where do you recommend buying it from? There's a lot of good sellers. I buy most of my hair off Ebay. Two good ones: BeautyExpress Ltd. UK and Sundance Imports. (I'm too lazy to find the links, sorry!)
what type is best? Probably Remy if you're going with human hair, but it's all so subjective. You may love synthetic. You really have to figure out what you want and how long you want to wear it and then go from there. Do as much research (archives on this site) as you can before buying anything.
Glad I could help! Please do major reading on as many sites as you can find before you make up your mind. You'll be happy you did. 