My '72 El Camino still has pretty good paint, but needs some touch up on a few nicks and a couple of small flake off areas. I bought it 4 years ago and don't know what paint was used when repainted last. What kind of paint can I use that would stick and not cause a reaction to the existing paint? It does appear to be a basecoat/clearcoat
and is not original paint. I can see places in the doorjams where bits have peeled off. Oh, and it's metallic silver.

Generally you won't have any problems doing a repair over unknown bc/cc systems if the materials that were used are of good quality. Stay with good products, PPG, Dupont, Martin Senior, etc. Sikken's products do have very hot solvents (almost like laquer thinner) and can be a PITA when doing repairs as they tend to lift more products than most. Spray some test panels to verify you have a blendable match before spraying the car. Air pressure and spray technique plays a big role in how silver looks, generally lower pressure and wetter coats will give it a darker and more colored look while higher pressure and thinner coats will give more reflection and a lighter color. Give your basecoat a good amount of flash time between coats and before applying your clear-this way more of the basecoat solvents will escape before the clear is applied. The longer the solvents remain in the basecoat that has been applied the more chance they will lift the original clear they are being applied over. If you clear to soon without the proper flash time there is a chance you'll trap the solvents allowing them to soften and lift the old clear. Hope I made sense, 95% of the time there will be no problems.