The decals will work just fine, you just have to do a little work to get them on right.
First, paint the area with a gloss clear finish to make it smooth. I use future floor wax at 100%, but any clear gloss is fine. Cut the transfer out with a sharp knife as close to the edge as possible.
After the gloss is COMPLETELY dry, soak the transfer in water only long enough to loosen it, then slide it off the backing paper into position. If you need to move it around, use a paint brush soaked with water. Once in position, touch the edge of the transfer with a dry tissue or paper towel to suck the excess water away, the transfer will suck down. Do not blot/touch the transfer with the tissue. Let it dry some, but it doesn't have to be absolutely dry. It make not be completely flat, especially on a curved surface or over rivets, etc.
Now the key. You need to get a bottle of decal set solution, such as Solvaset, Microsol, etc. You can get it on line from Micromark or at any good hobby model store. Its inexpensive and last forever. Put a drop of solution on the decal, enough to completely wet it. Let it dry without touching it at all. The setting solution will "melt" the decal into the surface and make it lie completely flat. You may have to use the solution more than once with GW decals. I've had to do it three or four times. In the end the decal is completely bonded to the surface.
Spray your mini with a final finish of Testors Dullcote (a matt finish spray) or whatever clearcoat you prefer. The decal will be perfect.
You cant even see the edges of the decals
This squad even has two decals on top of each other for the symbol and number:
