Forgot that they carried 8 in the basic game. That is a good thought, considering the size differential between a modern transport and a galleon. Need to make them faster too. Should be double that of a galleon. The naval vessel speeds are really weird in the game. For the sailing ships, I can assume a base speed of two knots per movement point, but then modern ships are too slow. Convert them to two knots per movement point, and they really move out, especially on the smaller maps. Carriers, destroyers, and cruisers would have 15 to 16 movement points, and battleships 13 to 16 depending on what you used for your basic ship. Iowa-class ships would get 16, the other Treaty ships either 13 or 14. Transports would get about 8. On huge maps that is not a problem, on the smaller one, not good. You almost have to tailor them to the map size.
If you do modify the transport to 8, you should also boost its attack to 4 and defense to 6, so it will not be sunk by galleys. Speed could go higher as well, but not as fast as the regular naval ships.