If you look at a line of speakers from most manufacturers, you'll notice that large floorstanders aren't necessarily better than the smaller bookshelf models, they often just go lower in the bass range than the bookshelves do. However, if you plan on using subwoofer(s) to handle the bass range, you really need to consider whether it is worth spending more on a large floorstander for a feature (deeper bass) that you might never take advantage of.
With subwoofers in the system, bookshelf speakers are a viable option. One of the advantages, due to lower price and smaller size, is that they allow you to use the exact same speakers at all 7 locations in order to have consistency all the way around. This is helpful when a car goes from left to right or a plane flies from front to back, because the sound remains consistent.
If you can't do 7 of the same speaker, then at least try to do 3 of the same speaker up front. Even in this day and age of surround sound, the front soundstage remains critical. That's where your attention will be focused, whether watching a movie or listening to music. And if you can't do 3 of the same up front, then at the very least try to find a centre speaker that sounds as close as possible to the left/right speakers. Bookshelf speakers make all of the above easier.
Were I in your place, I would seriously reconsider your plan to buy two floorstanders and a centre speaker, instead opting for three of the same bookshelf model up front.
