OK, I know I’m in the minority here (Hector, chime in anytime!), but just to weigh in on the other side of this question- I really do prefer a well-chosen liqueur added to mousseline, even in addition to chocolate or fruit puree or curd. It improves texture/mouthfeel and adds a layer of complexity to the flavor that you just can’t get with a single note of chocolate or fruit. And I have never had a mousseline break when adding both. I usually flavor the buttercream a day ahead to give the alcohol some time to mellow.
When something other than the liqueur is to be the primary flavoring, I don’t use the whole 3 oz, but rather use a smaller amount and taste as I go when adding it. I stop when I can just taste a difference, but still can’t quite make out the secondary flavor (liqueur or extract or whatever) on its own.
Chocolate is enhanced by Kahlua (or a little instant espresso dissolved in water or vanilla extract), Frangelico, Grand Marnier, or cognac.
Vanilla extract is great to combine with chocolate, and/or coffee, or apricot puree, but I would not combine it with any fruit curd. For Lemon curd, add Limoncello to taste (again, use less than the full 3oz to let the curd flavor shine through) and for orange curd, use a little Cointreu. For raspberry, I would look for a qood quality raspberry eau de vie or chambord, and for strawberry, I would just leave it as is, except maybe add a little of La Cuisine’s wild strawberry essence (or do as Hector does, and use Cordon Rose strawberry conserve for both the puree and the liqueur). If you want to add extract to raspberry and strawberry (not my choice), use almond or vanilla very sparingly- keep it in the background.
One of my all-time favorite mousselines is hazelnut, which I flavor with Frangelico, hazelnut paste, vanilla, and cocoa butter. Lemon curd mousseline is also wonderful, as is apricot-vanilla.