hi hayley, dont worry about adjusting the ph, 7.2 is fine, more important that its stable.
try your best not to feed them much at all, and make sure all is getting eaten, uneaten food will quickly pollute your tank.
your tank will be cycled in about 2 or three weeks and things will get better.
unfortunately the guppys dont cope well with sudden changes, although will live happily in a range of temps/ph. stability is the key.
i would suggest some hardier fish if you have no luck with the guppys, maybe some danios, or maybe kribs, which i have found virtually indestructible
hope things get better for you