1 The shape is fairly irrelevant, just immerse a tiny bit in the DW. The idea is that a flat strip is easy to make and you would find it difficult to get a piece of pure Gold any cheaper. This is my chosen method, for that reason. You could roll out a small coin if you prefer. If, for some reason, you really want a wire shape, then it would be fairly simple to drill a hole in a piece of hardwood and pour molten gold into the hole, thus giving you a short rod. I can't see the point of going to that effort. A strip is quick and easy to make. Once it is thin enough, you could even use tin snips to cut off a narrower strip, say about 3mm wide.
2 You are not correct about having to buy the Cookson Gold wire in a full reel. You can buy any length of the wire, 5 or 10 cm, for example. The reel size is for bulk buyers as it encourages a buyer to take a whole reel if their chosen amount is close to a complete reel. That way, shipping is easier for the company, as they just drop a reel into the packaging and don't have to cut it. The last time I bought from them, I bought 15 cm.