I personally think you were given the wrong advice about switching. Switching is more likely to keep her interested and increase milk transfer. Babies are likely to feed for longer when milk flow is the fastest and although 'hind milk' is important, it's a bit of a red herring really and if you swap sides each time the sucking slows down, then back again, as many times as you need, she is very likely to get more milk. It will also increase your supply to 'switch nurse'.
Also, when you say your baby is still on the 6th centile and has only gained 8oz in a month, do you mean she was born or or followed the 6th centile and has gained 8oz from her birthweight? If so, then allowing the two weeks to regain her birthweight, 4 oz a week for two weeks isn't that bad at all and is within what is classed as normal, although the lower end of it. Some babies have to be on the 6th centile, that's how it works. 50th isn't a 'target', it is an average, so some babies have to be on the highest and lowest centiles.
Can you attend a La Leche League meeting or similar in your area? They are likely to be able to give you the best advice on where to go from here, or indeed if there is even a problem. If your doctor hadn't weighed your baby, would he/you think there was anything wrong? Is your daughter otherwise well and thriving?
I would personally be reluctant to supplement with formula partly because it does remove some of the benefits on BF and partly because it is not condusive to keeping up supply and it can also be a 'slippery slope' to FF.