Repotting always shocks a plant a bit. New top growth just indicates that your plant has forgiven you for disturbing it and is ready to go on growing. You shouldn't fertilize a plant that isn't growing, just like when they are dormant in winter you don't fertlize them and ease off on the watering until the weather warms up and they show signs of waking up.
The Osmocote fertilizer is not meant to be dissolved in water. You mix it into the top 2 or 3in. of soil. It is coated little pellets so it doesn't all release at once. It dissolves slowly over the course of months when you water the plant. Always read the directions on the package before you put any chemical on your plant!
I don't think the temperature really matters, if the soil is dry you should water them. Epsom salts (magnesium sulphate) solution is good for most plants, but you should check if your soil already has magnesium in it. About a teaspoon per gallon of water should be plenty. Magnesium is what is known as a "trace" element that plants need, but they only need a trace.
