Bear with me here and I'll try to answer everybody's questions ...
Snyders - I just went basic black with my cart, so it goes with basically any colour goat (in it I have had red, white, black and tan) and sometimes if I want to dress it up I'll use different coloured pillows or throws over the seat. I'll take some pictures later today.
My boys should mature out about 165 lbs, give or take a little. Going by the one and half times his own weight rule that means when mature they will be able to pull 247.5 lbs. My cart weighs around 55 lbs and then I weigh 114.6 lbs. So yes, they can quite happily pull me in the cart. I guess I am pushing them a little now because they are not quite at that mature weight yet, they would be only around 140 lbs. I also push them a little when I drive them with myself and another adult in the cart - that takes it over the 247.5 but they seem to cope - I dont do it for long trips though. Long trips are strictly me, or me and a child. Actually most of the work my boys do is childrens rides, but they are quite capable of pulling adults. Driving the pair together they will be able to pull 495 lbs; I'm having a light four wheeled vehicle built for them to drive as a team, and carry a few passengers.
Your does - at 2 yrs old - seem rather small at only 80 - 85 lbs. Thats around 35kg (bear with the metric here) and my boer does usually hit 30 kg around 4 - 5 mths. My 10 mth old I just showed was 51.5kg. Just bear their weight in mind, make sure your cart is a light as possible, and start off with short sessions so as not to overwork them. Also - proper feeding is important - if you are going to be working them, they need to be fed like a working horse. My wethers get a kilogram of grain each per day, normally wethers only need hay but when they are working they need the grain.
ThreeBoysChicks - I had an old mini horse sulky, I just took to my welder and got him to copy it but make it smaller for the goats (width between the shafts and length of the shafts). I'll take some pics later. My sulkies cost $AUS500 and the leather harnesses about $AUS250. The harness is for a mini horse, altered by hand to fit a goat.
Farmgirlie - start now with your kids - teach them to walk on a halter/collar/both and to tie up. Teach them voice commands - basically to stop, go, left and right. I use walk on (go), steady (slow down), woah (stop), gee (right) and haw (left). I also teach them hop up - to jump up onto something, and hop down.
Chirpy - I've worked does in harness before, not a problem. If they are lactating the udder can get rubbed by the harness, so you have to watch that. Some people say they wont get as big as wethers, but if you feed them properly, they will. You shouldnt work them while heavily pregnant or lactating, so that restricts the work with a doe. But if you never join her, its just like having a wether. There is an advantage using a doe as a pack animal, particularly if she is in milk, because she carries the milk for you
Does can sometimes be more grumpy than wethers, but thats not always the case. Nubians tend to be more stubborn and strong willed than other goat breeds, and so are more of a challenge to train but I have seen them as very accomplished working goats.