The Etudes are single target spells. You don't have to worry about others being in range.
What I'm wondering is what determines how much they increase a stat by. At lvl 36, max singing/wind/string skill, with a Rose Harp +1 (+2 Etude) this granted me either +6 or +7 to a stat. But without the instrument, it only grants +3.
A lvl 60 BRD was giving my WHM +9 MND with this spell (dunno the instrument, but he wasn't high enough for the 2nd tier Etudes).