I wouldn't worry about any of them dropping a bit of weight, as they look like they could all stand to lose a bit. Not saying they are much overweight either, but they look on the heavy side of 'good'. Horses don't need to fatten up to cope through winter, like bears, for eg. It isn't any healthier to fatten them up for winter as it is for us to do the same<G>. They may eat more hay in the colder months though, as digestion of fibre produces heat.
I don't know much about beet pulp, except that it seems popular at this point in time & there's not much in it, nutrient-wise. It is high fibre & *can be* very high in sugar. I agree with sillybunny that I wouldn't bother feeding it, unless they weren't getting enough other fibre - say they had a prob with hay for eg.
I would be feeding the horses hay, with a *good* complete supp to provide the necessary balanced nutrients they were missing. If they don't get any green pick, I would also keep feeding the alfalfa cubes(I'm assuming they're just alfalfa, nothing else?). I don't know the Reliance 11 pellets, whether they're a 'ration balancer' or just a pelleted feed. If they are grain-based, I'd be hesitant to feed it to the ponies, or any horse prone to lami.
Regarding what exactly you need to supp them with for balanced nutrition, depends on their environment & what's in their feed, so an analysis of the hay would be helpful. There are programs/services such as FeedXL.com which I find great to do a basic diet analysis and analyse & mix & match feeds to work out the perfect diets. Unfortunately what they tell you on the feed package is not always accurate.
I'd be playing it by ear regarding weight with your STB. Add some oil, sunflower seeds, eg, to her feed if you find she's not doing well on what you're currently giving her.