Hi and welcome
Obviously you have found a really good sales assistant at p@h, which makes a welcome change! We usually hear much more of problems from bad advice.
You may find these two guides here helpful:
They tell you about the weekly weigh-in and how you can judge at any age whether your guinea pigs have a good weight/size ratio since guinea pigs have such a wide variance in sizes and weights.
The second guide talks you though the whole body for you to learn what is normal and what not. To spot what is wrong, you have to know first what is right.
The Importance Of Weighing - Ideal Weight / Overweight / Underweight
Guinea pig body quirks
We have got lots more really helpful guides for new owners on a wide range of subjects (settling in and making friends; understanding behaviour; housing, care and diet; family pigs; health care; recommended vet and rescue links) which you will hopefully find very helpful. The collection comprises answers to the most often asked questions by new onwers as well as providing important information so you know what to do when things don't quite go to plan and to prepare for the inevitable vet care.
New Owners' Online Starter " Booklet" For Really Helpful Advice