Mita, my favorite frosting for piping fill stars on 3d bears is the mousseline. Be sure it is executed correctly because a thinner mousseline will be a bit too runny for stars. Use the chocolate variation if you wish. I wouldn’t recommend glazing the bear prior to fill in possible gaps from piping stars, a very valid concern. To fill gaps I use either 2 methods: with a toothpick I swirl between the gaps, or with tip #2 or so I pipe little dots or you can use a small french opening tip, too.
I’ve done bears with buttermilk country cake, golden genoise, and chocolate genoise. Elicia loves using the Golden Luxury cake. I like to refrigerate the cake well so it becomes sturdier, specially when handling the piping or during transportation. For added strength, I leave the center core, or fill it with aluminum foil or cardboard.