This will definitely be a good start. The plant may come in a little small, but it will grow to be good cover. With how many guppies you will produce, as ugly as it sounds, sometimes survival of the fittest isn't the worst.
Yeah, my LFS asks for them around 3-4 months old so they have some good size and to maximize the amount of time the customer gets with their fish. Since you will be colony breeding like I do, you will find that selecting/netting is harder than it looks! I usually let the store owner know that there may be a couple that snuck in younger or older than what he was looking for, but I don't include those in my counts.