Epoxy is fine over fiberglass, you will not have an issue there. Fiberglass is very porous if you sand all the way down through the existing paint, so it may look better if you add an extra coat to the fenders in these areas.
Single stage urethane looks better and is much more durable than acrylic enamel. Think 1990s technology (urethane) vs 70s technology (enamel) or CDs vs 8-tracks. Enamel is very brittle and wont stand up well over time to any flexing the trailer may do as you load and unload the boat.
One important thing I left out above is to clean, clean and clean again before painting. Paint can only stick to what is underneath it so you want to get all the dust cleaned out and any contaminants off the surface before you begin painting with the epoxy primer. You will need a good water-based cleaner (soap and water or dish detergent work fine for this project) and you also need a solvent-based wax and grease remover. If you touch bare steel with your hands/fingers, you will leave oils from your skin behind that only wax and grease remover can get out. Water based cleaner gets the other contaminants off that wax and grease remover won't get.