I agree, so to be more specific the pony is not a part of the main gas plan. It is sized or as you say, planned, to be sufficient for the depth, and other considerations, of a recreational profile dive.To be a pedant (moi?), you need to plan that the Pony contains sufficient gas to get you out of trouble. Meaning you plan to need it when all else fails at the worst possible time. Therefore the the deepest point and you've a "safety stop" to do (not deco, oh no). You will be stressed so need to use a stressed SAC and allow a minute or two to sort your act out.
You're right that this isn't part of the primary gas plan, just the backup -- bailout? -- gas plan.
James