I am new to plane modelling but I know I would like to replace some decals that come with kits. How do I choose? I like the schemes and colours of some but they say they are for a specific variant. Does this mean only the e.g F-15e had these markings or can you mix and match? Do they make genric decals for f-15's for standard markings such as "no step" etc?

Hi Godfather
It all depends on if you want to stay authentic to the version of the aircraft you are working on. And possibly on what the difference of the variations of aircraft are. Some of the variations are just different armament and ordinance but some variations in aircraft might be different lengths of wings or lengths of cowlings etc...
Obviously, as long as you stick with the scale of the plane (1/48th, 1/72nd..etc...) You could probably use any of the variants for the aircraft you are working on. But again, If you want the plane to be 'historically' authentic I would stick with the decals for that variant of aircraft.
In the end tho...You are making the model for your fun and enjoyment and there is absolutely nothing wrong with mixing and matching decals to make it look like you want it to look.. :-)

Hope this give you a little insight and I am sure others will come by and give you their views and advice.
Good modeling,
Wayne

One thing I do when loking for decals is to do a search on the Hannants.co.uk web site. You can actually see pics of the decals that are available even if you don't buy them from Hannants.