As a 29 gallon, I am assuming the tank is around 3ft? If you have used stuff from a running tank, and only added 4 platies, you may not see a rise in ammonia or nitrites. If the 4 fish survive a month then add another 4. If all right after another month, add another 4 until stocking level is reached. Actually, if some of the filter media is from an established tank, you could probably add 4 more small platy sized fish now.
Don't worry about the pH, the fish will be used to local water conditions and, you are right, better to leave it a bit high than try and alter it. 7.6 isn't too terrible.