You will have to wait and see what she is like, but I would start of giving her just a teeny taste of chaff, enough to mix the vitamin and mineral supplement, and then wait an see if she needs anyhting else, I think it will be unlikely. also the amont of hay she will need will be absolutely miniscule compared to what you are used to looking at, and if it is good hay, you may well need to soak it for at least 12 hours to remove the goodness. I feed my highlands be eye and feel, but weigh our shetland's hay if he is needing any, because 1.5% of his bodyweight looks such a pathetic amount, and it has to be less than that if he is getting grass too. If he gets a 'hard feed' it is one of my two year old son's tiny handfuls of Top spec Top Chop Lite. And despite all of this, he is still fatter than I would like. The pleasure he gives us makes him worth all the work, though, I'm sure your shetland will be the same.
I would speak to westgate Labs or similar about what worm eggs counts and so on should be done, and that way you won't be giving her any chemicals she doesn't need, and you will also be sure any you do give her have worked and there is no resistance issue.