The English Horn is a fairly late addition to the orchestra, and so is used either as a solo (for example in Franck's Symphony which has a famous English horn solo in the second movement) or added to the oboes playing whatever the composer decides he wants. There is no really correct way and it is completely up to your judgement. Naturally, if you simply double the other instruments it will be more boring and less rich sounding harmonically.
Trombones are also a somewhat late addition to the orchestra concurrent with Beethoven. In the 5th symphony you can hear the effect of the trombones being added to the last movement, to give more power to the lower range in general. They don't really simply double the cellos or bass, but are added when more emphasis or depth is needed in a strong fortissimo as Beethoven does there. But in later orchestras, there is absolutely no necessarydoubling with cellos or basses (unless the music is very simple) and the trombones are simply used for their own tone quality and to give a harmonic basis in a large orchestral tutti, supplemented by the added bass of the tuba. If you listen to Mahler's symphonies, you can hear great use of trombones both as solo (int eh 3rd symphony in particular) and in general scoring, for their timbral effect as well as reinforcing a loud section. So this all boils down to there is no "correct" way to use them, but simply remembering how they are very powerful and would naturally be used to play lower harmonies in fortissimos, though also have beautiful timbral qualities in quiet sections, either as a "chorale" like effect ( as in the Brahms first last movement the first occurence of the trombone ensemble playing the beautiful chordal section) or soloistically.
