You're right, Legacy's larger speakers always seem to get the most press! But before I graduated to the larger speakers, I started out with Studios first, and they are fantastic bookshelf speakers that punch WAY above their size.
I have not personally heard the Expressions, however they have the same spiral ribbon tweeter that are in my Phantom surrounds, and the same 8" woofers that are in my Studio HD rear surrounds, and both of those speakers sound great in my system.
The spiral ribbon tweeter has a wider dispersion zone than the AMT tweeters, so that will give you a wider sweet spot. The 8" silver-graphite woofer is a high quality driver with a massive motor structure and long excursion. The Expression speakers will do a great job for you in a small system. Bass extension will get you down to a solid 40Hz, although I would probably crossover to the Metro subwoofer around 80 Hz, and the Metro will extend down to about 20Hz, in-room.
Yes, the Cinema HD is the small center channel. It was designed for tight spaces in AV cabinets, but it can also be used on shelves. It weighs 18 lbs, and would be a good match for the system you are proposing. The larger Silverscreen HD center weighs 42 lbs, and while it will sound better, it would be too heavy for your shelf.
Overall, I think what you are proposing would be a very nice, well-balanced system. You might want to consider some surround speakers too, if you are going for home theater. The Studio HDs make great surrounds, as well as the Deco, depending on where you need to mount them. If you can find a way to make the Silverscreen HD work, it will make all of your movies sound much better, since the center channel really is the most important speaker in a HT system.
Feel free to ask any questions, and I will help you out any way I can.