It's hard to say without knowing more. What's the temperature inside the rice cooker during the 15 minute resting phase? How big is the rice cooker? How much salmon are you adding, and what will that do to the temperature inside the cooker?
Ultimately, you want to get the fish to its target temperature, but you don't want to exceed that temperature. If you've ever poached fish, you'll probably know that it's the temperature more than the time that affects the outcome. The trouble you face with the rice cooker is that its temperature while cooking is probably much higher than you want to cook the fish to (salmon is "done" somewhere around 135°F, but steam has a minimum temperature of 212°F). Once you turn the cooker off, its temperature will start to drop and keep dropping until it reaches room temperature. Adding a mass of cold fish is going to speed up that process, so the amount of fish you add will make a difference. A larger rice cooker will have a greater amount of latent heat, so that makes a difference too. And taking the lid off the cooker periodically to check the fish only adds to the degree of difficulty.
I think your best bet is some educated trial and error. For a small amount of salmon (~two servings) in a medium-sized cooker, I'd add the fish a few minutes before the rice is done cooking and then leave the lid on for maybe 10 minutes after the rice has stopped cooking. Check it at that point, and if the salmon is done the rice will probably have rested enough to be tasty. If the salmon isn't quite done and the temp inside the cooker is still above the target temp for the fish, put the lid back on (quickly!) and let it sit for a while longer. If the salmon isn't quite done and the temperature in the cooker is close to or lower than the target temperature for the fish, finish the fish in a medium (say, 300°F) oven for a few minutes.
Remember that you're dealing with a number of variables. Be sure to write down the times and quantity of ingredients each time so that you can refine the recipe, and be aware that changing the quantities may require changing the times too.