Phloxes are known as lime lovers. I am planning to mix peat into the soil. How will Phlox react to the acidic peat?
If the phlox are in the ground, digging in the peat would be more harmful than the peat itself. ;-)
If you already have an established bed of phlox, then you would need to dig up phlox plugs, protect them from drying out and then quickly prepare your bed with the additions of peat and lime mixed into it.
But I am wondering if this is too late in the year to do that.
As far as your question about any negative affects of adding peat, you first would need to know how acid, neutral, or how negative on the acid scale your virgin soil where the bed will be made already registers.
I have always thought of peat as not having a very high acid level, but I have never researched to see if that is true or not. I do know that a prepared bed with only peat for my azaleas will cause them to suffer due to that bed not being acid enough. With my azaleas I have to also add shredded pine bark mulch to the peat beds before my acid loving Azaleas will prosper. This is the only reason I consider peat to not be so high on the acid level scale.
Of course Phlox prosper around concrete sidewalks which are negative on the acid level scale, so what is the reason you are wanting to add peat if you have not first researched to know what level of acid that peat will or will not add to the bed where you want to plant the phlox?
I grow creeping Phlox in acid soil with no problems. I would think that good drainage would be a more important consideration.
Cultivated phlox are quite adaptable regarding soil pH. As katrina1 says, the effect of peat will depend on the natural pH of your soil, and also its base component (clay or sand). Clay will have a high buffering capacity. Sand will not.
If katrina1 was growing azaleas in sphagnum peat, the contention that the medium is not acid enough for azaleas is erroneous. The problem must be something else. I suspect since sphagum peat has almost no nutritive value, there was probably a deficiency of one or more nutrients, or an imbalance of nutrients caused by high acidity where the pH may be as low as 4. Sedge peat is also sold, with varying and much higher (less acid) pH. Sedge peat may not be acid enough for azaleas.