Hiya! Okay some answers to your questions. If they can't remember, worm her. It will take alot to over dose and harm the animal. Wormer is not expensive. We have an Orshelsns store that sells a tube for $6, it works as good as the ones for $15.
Worming her doesn't effect the other horses in anyway. They all have their own schedules probably. Any kind of wormer will do the trick. Get the paste in the tube, really simple. =]
Depending on the wormer, I have had the vet say every 3-5 months.
Actually, if you have a Weaver brand halter that fits Lacey, it will say 'Small' horse and give an estimated weight. Wormers don't have exact weights. They go like 500, 750, 800, 1200.
She probably weighs in between 600-800 pounds, so just give her 750. Giving them a little too much is no big deal.
Hope that helps!
Lettin' the cat outta the bag is a whole lot easier than puttin' it back. -- Unkown