Thanks I just used wood glue .I don't have anything bad to say about a brand called Renia from Germany (http://www.renia.com/englisch/msds.html). In particular, the Colle de cologne glue commonly called CDC in the trade is hands down the strongest bonding glue I know of. Its also what I've seen Viberg use before in their shoemaking videos (I expect they don't use it for heel pads however as they are always coming unstuck). I find most other glues to be more of a placeholder in the fact that while it will still hold for a while it also relies on other methods to help such as sole stitching or nailing heels etc. The other thing about glues from brands like Fiebings as well is that they are generally for leatherworking with things like wallets etc, where you use them lightly to hold the pieces together until you hand stitch. I don't know of any cobblers that would be using glues like that on day to day work. CDC is the only glue I would use when the glue is all you can use, for example in bonded soles or when heels can't be nailed to further secure etc. Be mindful however that CDC comes in larger quantities, 1L 5L etc, which for anyone doing at home leather jobs, is probably way more then whats needed. You might be able to get it in smaller quantities but I've never seen it.
However, for general purposes, Any leatherworking glue is more than efficient for a heel pad. Its something that isn't going to be terrible long term if it comes unstuck you know? So for your heel pad I would say feel free to go with the Fiebings, youll most likely have no trouble.
To get the best out of it, keep something on top of the pad after you've applied 2 coats (Leather likes to soak in glue), that will help keep pressure on it from top down. I would suggest something like a full water bottle. These glues like pressure (Which is where you see things like sole presses in shoemaking videos) and will benefit greatly from something weighing it down while it dries. Place it in immediately after you've applied the glue while its still wet as you'll be able to slide it around to the correct position, sometimes when the glue dries it will stick immediately and you might be left with a heel pad in a bad spot. Hope that helps!
I know this got a bit long-winded but hey I thought id just put it out there. Sorry if I rambled on a bit haha