Whenever selecting an instrumentation to arrange/compose for it’s important to consider the following:
Let’s explore each question individually.
This is a relatively simple question, but it can be a fundamental consideration when arranging music. If the source material features a soloist the easiest choice would be an ensemble formatted for a soloist (soloist + piano, string quartet + soloist, etc.). Conversely, if the source material is an ensemble piece (the melody is performed by multiple instruments) it would make the most sense to avoid choosing a ensemble formatted for a soloist.
It’s important to note that these are guidelines and not rules. It is certainly possible to arrange a solo for a quartet and visa versa.
A homogenous timbre can be achieved by using ensembles comprised of the same instrument or instrument family. A string quartet is a relatively homogenous timbre because all of the instruments are bowed string instruments, whereas a wind quintet is a mixed timbre due to the different methods of sound productions (flutes, single reed, double reed, and brass).
An ensemble with an homogenous timbre can often be more forgiving to orchestration errors such as mismatched tessituras or confused dynamic ranges, however they can also limit the creative possibilities for your orchestration. In other words, when writing for a string quartet it’s slightly less likely that you’ll cover up your melody because of writing a part too high or low in the ensemble’s range, but the same isn’t true when writing for a mixed ensemble featuring woodwind and brass. At the same time, a mixed ensemble provides more opportunity to try novel instrument combinations.
This all important question is one that many professional composers start their compositional process with. The entire scope of a piece can (and probably should) be informed by knowing who will eventually be the one performing it. No two musicians are alike and by knowing your performer in advance you will be able to play to their musical strengths and avoid their weakness.
An extreme example would be composing for a young musician that only knows a handful of scales (let’s say C major, F major, and G major). A wise composer would choose material that works well in these keys and doesn’t make use of too many accidentals as that might confuse or discourage the young musician. It’s important to know who you’re writing for.