Like you, I switched from a fixed gear to a Shimano internal gear hub. My fixed was set up with 44/17. I swapped over the entire crankset and used the supplied 21t Shimano cog. The old 44/17 fixed fell must about in the middle of the 44/21 gearing of the internal gear hub.
Keep in mind that if you were to use a 16t cog with the Alfine, you would only be able to install it dished outward. Only cogs larger than 19t can be set up dished inward without removing the drive side dust cap.
Smaller rings and smaller cogs wear out faster than larger rings and larger cogs. Rear cogs are smaller, so they wear out faster than chainrings. So you should try to use the largest rear cog to achieve the appropriate gearing. If you're thinking about trying a higher gearing, Usually the hub is supplied with both a 19t and a 21T cog. Your best bet would be to try to set it up with the 21t cog first and see how it works. If it's too low, you can use the 19t cog, and just shortening the chain. If you try the 19t first, cutting the chain to the appropriate length, then find out it's too high, the chain would be too short to use with the 21t cog.
Last edited by vredstein; 07-20-09 at 01:25 AM.