As long as the system is completely flushed, yes you can use it. Instructions should come with the Summit coolant if there are specialized steps to take.
In hotter temps, you can lower the amount of coolant in the mix somewhat. You can run all the way down to 30/70 (30% coolant). You can add Redline Water Wetter or Royal Purple Ice or similar products. You can also have your tuner change when the fans come on and for how long. Combined with a colder TStat, that will really help keep temps under control.