Agree with NM. I'd keep it simple, start with the beet pulp(or ricebran, alfalfa, etc are some other healthy high energy alternatives) & see how she goes. If she picks up on that, great. If after a few weeks or more she's showing no signs of improving, then I'd think about adding something else. A little oil or copra or such can be healthy 'weight gain supps' to add if necessary.
Don't know that I agree with free choice minerals tho - have heard of horses apparently 'OD-ing', but haven't looked into it enough... but definitely think appropriate nutritional supps to 'fill the gaps' is a good move. FeedXL.com or consulting a nutritionist(independent of feed co's) will help you work out what your horse may need, with her specific diet. As you're feeding beet pulp anyway, you could just add a powdered supp to that, or feed a 'ration balancer' type feed.
Celeste, yes, a *good quality* 'balanced' senior feed may well be good. I personally don't think you generally get that with Purina though. And it depends what else the horse is getting in their diet as to whether it is indeed balanced. They're usually grainy/starchy, often sweetened, so it would also depend how little & often(or otherwise) you can feed as to how appropriate they may be.
I also want to comment on amounts. While 2lb may well be nothing for some horses, it depends what's in that 2lb for starters, but may be quite adequate or even too much for some horses/some ingreds. OP is also the new owner & recently introduced this feed, so small amounts are vital to start with. While you may well want to feed 6lb or more daily, that would also depend on how often you can feed - I wouldn't feed more than about 2lb per meal, max.
As for your QH, PM, if she's a good weight, doesn't sound like she needs extra than grass/hay. If she's an 'easy keeper' you may have to monitor her intake & maybe even restrict it. I'd opt for a powdered or very low dose pelleted 'ration balancer' for her, to give her the necessary nutrition without adding substantial calories.