Pariahs are excellent guard dogs as well as watch dogs. Yes they are generally territorial but since there isn't much of a standard for them not all of them are the same. I've seen Pariahs so fierce that they will take on rotties and also some pariahs who well, just don't care :)
If you want an alarm barker get a hound as they are smart enough to get along with people and other dogs but they become alert and vigilant at night. This behaviour is because they were bred to hunt and hunt in packs with other dogs and handlers and also have a great sense of smell, hearing and the most amazing sight.
Im talking from experience as my bitch Xena is a Chippiparai and she is almost a year old and is a great watchdog (barks and becomes alert at any sound or movement) but at the same time is friendl enough with family and friends.
Overall it depends on how you socialise the pup. More socialising = more friendly.
Hope I helped. Regards, Swaminathan