When dealing with older decals, I have found that it can often be hit and miss when using MicroScale Liquid Decal Film. Sometimes the liquid does its job and the decal is useable. Other times, not so much, as the decal still falls apart when released from the backing paper. I guess it depends on the age of the decals or maybe the manufacture or both, not sure. I would find a decal on the sheet that will not be used and apply the liquid to it and see how it reacts before coating the whole sheet.
You can always clear coat the decals as magicmustang states in his post but do it in very thin coats as the paint will add thickness to the decals. You will also have to cut out the decals very close to the design as the clear coats will cover the entire sheet.