Any kind of grass ok?

I wanted to get some timothy hay at a feed store, but my mother complained that guinea pigs don't need a special kind of hay. She said the grass in our backyard was fine since we haven't treated it with any chemicals or anything of that nature. She also said they could survive on the rose leaves we have in our yard. On every website I always hear that timothy hay is the way to go. Is it safe to also use fresh grass as long as it is untreated? Or should I just stick to timothy hay? And also, is there a special grass that maybe I could grow in a pot for my GPs?

Miss Piggy