Usually potting soils only come with a small charge of fertilizer, so it's likely appropriate to supplement additionally. What did it say on the bag about the fertilizer your soil contains? Did it say you wouldn't have to fertilize for x number of months, or did it just say that it contains slow release fertilizer? FWIW - 'slow release' doesn't necessarily equate to 'controlled release'. Granular fertilizers that are marginally soluble are 'slow release'; also, organic soil amendments like hoof/horn/feather/cottonseed ..... meals are referred to often as slow release fertilizers.

Al