I wouldn't be worried about this in the slightest. Both the Foster and the Smith are just getting up in age and want to spend time with their kids and their kids' kids I'm sure.
The absorbing of a company like this can be a very good thing, for us, Petco, Dr Foster and Smith, and Liveaquaria. It will bring more product into the market much easier. This has been something I have already been starting to see at my local Petco, they started housing coral for sale. This can easily lead into greater and easier availability of corals to the middle of nowhere people like myself.
Keep in mind, Petco is a business. They are good at it, if we like how or why they do things or not. They will do things in a smart manner. I would easily suspect the normal foster and smith stuff to be absorbed into normal Petco wares. They will keep liveaquaria as it's own entity, because it is specialized and something that Petco doesn't already have.
Overall, I'm excited to see what happens. It'll end up saving me money on shipping costs.