No need to beef up the tail for 3D (in terms of building). But a solid 70 ounce servo for the rudder and elevator will be fine (such as a hitec 605 or equiv).

Your 91 FX should be plenty of power for hovering the Yellow 60-size series. My preference is a Saito 100 or YS 91 just because I prefer four strokes - the two strokes are great engines. The Saito 91 will work, but much like the OS 61 FX, it won't hover and recover.

Squeeze, squeeze, and squeeze those engines in there!

-Juhan