Are you editing the original .rul file directly, or creating a mod directory with a new file? Also, where in the file are you making this change, that is, under what heading? It might be helpful to look at the ruleset for the XcomUtil Fighter Transports mod:
crafts:
- type: STR_FIRESTORM
soldiers: 10
vehicles: 1
battlescapeTerrainData:
name: FIRES
mapDataSets:
- BLANKS
- LIGHTNIN
mapBlocks:
- name: FIRES
width: 10
length: 10
deployment:
- [5, 3, 1, 2]
- [5, 4, 1, 2]
- [4, 3, 1, 2]
- [4, 4, 1, 2]
- [4, 2, 1, 2]
- [4, 5, 1, 2]
- [3, 2, 1, 2]
- [3, 5, 1, 2]
- [2, 3, 1, 2]
- [2, 4, 1, 2]
This mod might be as simple as replacing the above battleScapeTerrainData: with the corresponding lines from the small scout's definition, setting soldiers: to one and vehicles: to zero, and editing the deployment coordinates for your single soldier.