Composing in those instruments is mainly due to two reasons. Tessiture; guitar, piano, violin and bass simply have Tessitura: YES, meaning one does not have to look at the limits, comfort zones or limit to note holding while writing, it's a lot easier (which now that I have to write for an orchestra is hell, since I have to keep checking the piccolo's range and best sounding zone, and I haven't even dwelt on the wholeness of most of the wind section). The other one is midi sound, winds are probably the worst instrument sounds in midi, their sound is unnatural because it lack the usual modulation, varying strength and note transitioning. As such, hearing what you've just written be played out in such a way, may nudge you toward changing the instrument (like I did for Sandstorm, it's supposed to open with a trumpet, but a violin sounds better in midi).
You could try a modulation to either the relative minor of the key you are using, or using the minor of your root as the new key. Or you could run the minor theme transposed to your root's minor and ignore the key signature's accidentals and use your original root's key signature's instead (with some minor editing so that it doesn't sound rather off since it's not a perfect method [though very useful for some jazz renditions]).
For the regal sound, it's normally the product of the brass being too high above the accompaniment in volume and pitch, if you lower the brass somewhat and heighten the strings volume and pitch (I assume since that's what the Pirates of the Caribbeans accompaniment and melody is made of). Also, percussion has a heavy roll in the sound of it, it has a march-line like cadence and it's what makes it sound less like a string concert and more action movie like. Making it louder could solve some of the issues.