One-shots and multisamples
A single note is enough for a stab or a quick melody. A multisampled instrument, with every few notes and several velocities recorded, plays like the real thing from a keyboard. Free orchestral projects such as the CC0 Versilian Community Sample Library and VSCO 2 Community Edition, university instrument archives, and piano sets such as Salamander provide multisamples for many instruments.
Playing samples convincingly
Write within each instrument's range and give players room to breathe. Vary velocity and timing slightly, use expression or volume automation for strings, winds and brass, and add one shared reverb so the instruments sound like they are in the same room. Root notes on the cards help you map one-shots correctly.
Licences for instruments
Instrument sample projects are among the most permissively licensed sounds available, often CC0 or free for any use, which makes them a safe base for a release. Recordings of public-domain pieces are different: the performance has its own copyright, so check the verdict on any recorded passage.