You could also try Allo Taxi and T'Leva (other taxi Apps). I have used both in the past. They're a bit hit or miss, especially if you don't speak Portuguese.

On the taxi mafia front, you could try getting the driver to meet you in the rental cars section of the car park, rather than in front of arrivals. Makes it easier for them to sneak in and out without being spotted.

I also like the tip from cargueiro on walking to KFC and getting a taxi from there!