F. Microcarpa is a vigorous and tough plant. It can also take surprisingly cold temperatures. I got my 2 from a local nursery that has since closed (Hollow Creek Bonsai), Fred used to let them get down to near freezing. I don't do that with mine, but I do leave them out pretty late in the fall, until temps are dropping into the 30s. I will bring them in at about 38 F and then put them back out in the sun. During the winter I don't bother with supplemental lighting and trying to keep them growing, I just put them in an unheated mudroom where temps generally run in the 40s. There are windows so some light, but we get almost no sun during the winter. They lose a few leaves but not many. Just need to keep them watered, it's easy to forget when they're out of the way like that.
Willow leaf ficus are nice plants but I think they might be a bit less cold tolerant. I don't let those get down below 40 in the fall and I keep them in the house in a window during the winter. They don't grow during that time but they hold their leaves pretty well. In late winter though the leaves do start to get ratty and I usually defoliate completely when I put them outside for the summer. I've tried keeping them under grow lights (metal halide or LED) during winter but haven't had much success with that.