I use All Mix as well and I always Ph my water to around 6.5. This has not messed up for the Ph of the soil. If you gave too much nutes, you will need to flush to get rid of the toxic salt buildup. That sound slike bad advice from your growshop.

Secondly, you don't need 2 ph testers. To test the Ph of your soil, ph the runoff water that drains out of your pot when you flush.

Now go flush those plants and work on the temps. To flush, give your plant 2-3 times the volume of the pot with ph'ed water. For instance, if your pot has a capacity of 2 lites, flush with 4-6 litres of water.