the sha-256 miners cannot be used to mine scrypt alt-coins. there are currently no asic's available that will mine scrypt coins but there is at least one company developing them (
https://alpha-t.net/). i have mined all types of sha-256 coins with asics, and all types of scrypt coins with cpus/gpus. lorenzo is correct with his statement that you just need to point your mining software to the pool of your choice. for my scrypt coins i use middlecoin.com because they mine the most profitable coin at the time and automatically convert it to BTC and pay you out every day.
edit - the sites that say LTC is that much more profitable is misleading. there is a greater power cost to mine scrypt coins. for the 2500 kh/s of GPUs i am running my power usage is between 1500 - 2000 watts. my avalons (4x102gh/s) use 3600 watts, and my antminers (2x180gh/s) use less than 800 watts.