The reason they suggest winter bedding plants is because you’d want to plant bulbs in the late autumn to bloom in the spring, and the winter bedding gives a bit of colour while the bulbs are growing. Once they finish the bulbs appear and you if you have layered you have colour trough to early summer. Then you can leave the bulbs in for next year and replace winter bedding for summer bedding.
I see no reason why you couldn't use the nasturtium, maybe keep it in another container now and pop it on top if the bulbs when you plant them in Oct and November.