I know this is an old topic here (Oct 1?) but just in case someone else comes this way I should warn you that using anything ohter then like materials next to each other is doomed to failure.
There is a condition known as "The co-effecience of Expansion and Contraction". Wherein it is explained this way;
All materials have a certain amount of movement. When one material had a certain density and it is applied next to a material of a lighter and harder density the two materials will move (expand and contract) and different rates.
This causes the two materials to seperate.
Therefore you should never use drywall mud, spackling or other none gypsum based materials either next to over a gypsum based material.
I know it will be a long time before someone reads this......but good info is sometimes hard to find!
Brian Ewing Ewing Restoration http://www.ornamentalplaster.com