My breastfed daughter hit a plateau right around a year old, as well. My pediatrician was NOT concerned since she has hit all of her milestones, was active, and eating a wide variety of healthy foods--this is also the time when we were transitioning from almost all breastmilk to including solids. Is she walking yet? This increase in activity can also lead to a levelling off in weight gain.
I agree with you about not wanting the sugar--not to mention the multitude of other ingredients that I can't pronounce in Pediasure. It just isn't necessary when you can feed her real food and have the same outcome. Premade shakes are for those without the time or inclination to do otherwise.
Try a smoothie of 4 oz. whole milk, 1/2-1 teaspoon Brewer's Yeast, 1/2 banana, and 1T peanut butter (assuming no allergies or family history). It is a kiddie friendly version of a weight gain shake we used to make for my little brother when he needed to gain weight for wrestling.
Also...remember avocados, salmon, lentils, canola oil for great sources of healthy fat, calories, and protein.