CPU mining for Bitcoins is almost never worthwhile since the advent of GPU mining. If your EC2 instances are free you're welcome to try but there's a better alternative.

A decent CPU won't get you but 3-5 MH/s of Bitcoin mining power, which at current difficulty and exchange rates will net you about 0.0046 BTC per day (worth about a penny at current rates). On the other hand, that same CPU will net you perhaps 3 kH/s of mining power on one of the sCrypt based forks, I'll use Litecoin (LTC) for this example. Mining LTC on that same CPU will net you about 3.06 LTC per day which, at current exchange rates, will net you about 0.0153 BTC, worth about 4.5 cents per day. It's still not a lot, but at present CPU mining on CPU-friendly forks is much more profitable than CPU mining Bitcoin directly.

As for whether your 50 c1.medium instances would be worthwhile, if (for the sake of easy math) we assume each can produce somewhere in the neighborhood of 2 kH/s of Litecoin mining (decent guess, a 1 MH/s BTC CPU tends to put out about 1 kH/s on sCrypt based variants) then that's a combined 100 kH/s worth about 102 LTC per day, 0.51 BTC per day, or about $1.53 per day. Since I'm assuming the cost of 50 c1.medium instances is more than $76.50 per month, I don't think it would be worthwhile for dedicated mining. If, on the other hand, you already pay for these instances for other purposes and the addition of mining is essentially "free" then an extra $76.50 (or 25.58 BTC) per month isn't too bad.

Finally, yes, all fifty of your instances could work together on a single account in a mining pool. This is so common it actually has nomenclature: each of the running instances would be called a "worker" and while some pools allow you to create many workers, that's really just for your own stat tracking and isn't necessary at all. Each instance can certainly submit shares under the same account without error.

You can use EC2 GPU instances now, but the price outweighs the earnings:

https://bitcointalk.org/index.php?topic=8405.0

As with all types of endeavour, you have to see if your earnings are bigger than your costs.

If you have access to those instances for free, you just have to figure out if your time to set everything up is worth it.

You should put your numbers in a profitability calculator, like this one:

http://tpbitcalc.appspot.com/

But from what I can see, you have under 100 MHash/sec in total, which would net you about 0.1 Bitcoins per day of continuous mining. Probably not worth the effort.

As for the pooled mining - the whole concept of it is that a lot of smaller miners contribute to solving a block. Mining with 50 or more machines is totally acceptable, but might be a bit of a work to get everything set up.