Cruiser bikes are definitely the most comfortable for the casual, short distance, cyclist. They are designed that way. They aren't designed to be comfortable in the long haul though. You'll probably start getting rubs from the large saddle, and your legs won't be in an optimal position for pedaling comfort and efficiency for the long duration. Plus they tend to be heavy (you'll feel it on anything approaching hilly), sacrifice handling for comfort with the handlebars, saddle, and headtube angles (fine for paths, etc. Maybe not so fine if you end up doing group/charity rides), and have limited gears (6 is nothing compared to 20 or 24 which are standard on hybrid/road/mountain bikes. You will miss that dearly on group rides, long rides, anything hilly, etc).

You shouldn't settle on your hybrid either though! Not all hybrids or road bikes are created equal, and they don't all fit the same. Especially in the growing class of "hybrid"/utility/urban/flat bar road bikes, there is a lot of diversity in build and fit type. Try out as many as you possibly can. Talk to bike shops and fitters about what can be done to accommodate your shoulder/back pain. Maybe also consider looking at touring bikes (more like a road bike, but with a more comfortable/upright geometry). A good shop can take the information you provide (short term goals, longer term goals, "I'd like to try's", and concerns about fit/pain/etc) and explain to you the benefits, disadvantages, and compromises of the bikes they carry that are close to what you are looking for (you may have to prod a bit though...). Take notes (on the bikes, how they ride, complaints, likes, etc. and the shops). Hopefully you'll find a combination (bike and shop) you are comfortable working with.

My guess (without knowing specifics, or being a doctor) is that you can find a hybrid (or maybe even an upright road bike, we don't all ride with inches of saddle to bar drop like the pros ) that is comfortable for you and still functional for your future goals.