I see two separate questions here from your post. But, in short, I'd call maintenance and follow their advice. If they say go, then go. If they say don't go, don't go.
1.
Is MEL 26-12-01 applicable?- While it is most likely that the light is illuminated for a leak, it is also possible to get the light because of faulty equipment (sensor, detector, etc) or because of direct sunlight on a hot day (I've heard of the first two, been told by an instructor about the third). For these reasons, you don't know whether the MEL is applicable or not, and only a chat with maintenance will let you know. Leave it to them to decide if it is applicable or not, that's what they're paid for.
2.
Is this a Go or No-Go Decision?- While the QRH doesn't say anything about altitude, the MEL has a series of restrictions you want to consider, should it be applied You'll be restricted to FL250 on your next flight, and you won't be able to enter known icing conditions (for this flight or the next flight), and you won't be able to use the APU at your next airport once the MEL is applied, so you better have ground power and ground air. This is now not only a talk with maintenance but with your controlling dispatcher about flight routings, altitudes, and contingencies, just in case. Depending on your jurisdiction, the flight dispatcher may still have co-authority over the flight on the ground (they do where I work until power is applied for takeoff), so they'll have input into your scenario too.