Ten nylon strings in five courses, and one of them is not like the others: the middle course pairs a string with another tuned an OCTAVE ABOVE it, which is why the registry describes this tuning as re-entrant. That octave is the sound people mean when they say a charango sparkles, and it comes from a single thin string.
Sets
Charango sets are sold as sets, and using one is easier than assembling ten singles because the gauges are matched to a short scale under high tension. Nylon is the standard; some players use fluorocarbon for a brighter, more focused sound and slightly less stretch. Either is a legitimate choice — what is not legitimate is guessing at gauges, because too heavy a set on a small carved top is how a top gets pulled out of shape.
The octave string
It is the thinnest string on the instrument and the one that breaks. Keep a spare. When it goes, the charango does not sound quiet so much as ordinary — the upper octave that made a full stroke shimmer is simply absent, and the instrument sounds like a very small guitar until it is replaced.
Change a course, not a string
A fresh string beside an old one is brighter and louder and drifts away from its partner within a day. On the octave course the mismatch is more obvious still, because you can hear the two pitches separately rather than as one thickened note. Change both, tune them to each other before you tune the instrument, and expect to do it twice.
New strings drift for days
Nylon stretches, and a fresh set on a short scale drifts audibly for the first two or three days. This is not a fault in the instrument or the strings — it is nylon doing what nylon does, and it stops. Tune often, do not chase it with the tuners mid-song, and give a new set a couple of days before judging how the instrument sounds. Run the open courses past the fretboard explorer once it has settled to hear each one on its own.