Decal solvents are a frequent thread on the "decals" forum, so if you do not get the answers you are looking for here, I urge you to try that forum.
That said, I start with the micro set. It is pretty benign. Only if the decal does not settle down well do I resort to a solvent (like Micro Sol). Decal solvents are pretty mean stuff, so once you apply them you have to leave the surface alone until it is completely dry. So- summary, start with Micro Set, only if that doesn't work go to the solvent. Use as little as possible, and use great care.