It would be wise to use a sealer as this would help to give a uniform finish. It would also help to guard against any lifting problems that you might have with the new paint and the old.
As for the decals under the clear, this I have done before and it helps them to last, along with making the car a lot easyer to wax.
If you are using a metalic or metal flake color it is a must to put a couple of coats of clear over the paint first. Other wise you run the risk of sanding the metalic when you wet sand to apply the clear coat.
I painted mine, let set for the time recomended by paint manufacture, 15-30 min., the applyed 2 coats of clear. I let dry 24hrs. wet sanded with 600 grit paper. Cleaned with final wash THEN applyed the decals, washed with final wash again, tack wiped then sprayed the clear. The first coat was a light, let set for 30 min then a wetter 2nd, let set for 30 min. followed by a wet final coat. I have found that if you put it on too wet and don't allow enough flash time between coats it can lift the color on the decal.
It does work, and adds depth to the over all look.