Thanks for your question. With regards to Magnolias, they generally like neutral or SLIGHTLY acidic soil. So, yes, you can use your acid food for them, but you may want to dilute it a bit from regular doses, and don't use it as often as the instructions say for true acid-loving plants.
As for Hibiscus, I've referenced some instructions from the following website, as they are excellent.
www.hiddenvalleyhibiscus.com/care/feeding.htm
"Hibiscus produce their most and best flowers with a fertilizer high in potassium - that is the third or last number in the formulas often given on fertilizer containers. Sometimes, people are tempted to use the "super bloom" type formulas which are very high in phosphorus - the middle number of the formula. This is NOT GOOD for hibiscus as research has shown that high phosphorus can prevent hibiscus from absorbing other nutrients it needs. These types of fertilizers may be sprayed on the leaves of the plant as a "foliar feeding" but should not be added to the pot.
If you use a local brand, it's good to check that the plant food is complete - that it contains "minors" which are small amounts of minerals that hibiscus also need. They will be listed on the side of the container. Look for ingredients like copper and iron. Be sure to follow the directions that come with the package of fertilizer. Hibiscus like to be fed small amounts often rather than large amounts occasionally. If you can hook up your water hose to a proportioner or fertilizer injector, you can water and feed easily at the same time. Otherwise mix your water and water soluble plant food in a container and pour enough of the solution into each pot so that some comes out the bottom of the pot. Fertilize when the pots are moist, not when they are dry. Use plenty of plant food in the summer and less during the winter."
At your local Sloat Garden Center, you will find that we carry a Hibiscus specific food that does meet these suggested requirements. And that is what I would recommend for success with growing yours.
