The advantage of the "intern" trade routes is that they won't theorically be caught by the barbarians during the early game but :
-- It may be better to use them to gain some gold instead as the beginning of the game is now more difficult on that part.
-- The amount of happiness needed for such an idea may be a problem but it depends on the level of the game you are playing as it may be a lot easier to find some happiness if you are playing the low levels.
-- I don't know what you call exactly "early game". If it extends to the point when you are able to dispatch some production to your cities, you may want to reconsider sending it instead of food, or one of each.
-- Crazy growth may create another problem : you will need some land to work when your cities will have many inhabitants. In the capitol, you will probably be fine but secondary cities may need you to buy them some land to work.
-- Last but not least, you should really reconsider the naval trade routes as they produce twice the amount of caravans. If you have to buy some ships, do it and use only one route per city and the three extra routes to get some gold to maintain your naval army. You will have then the same amount of food via trade routes but some troops and probably some gold with that, which seems to be a good idea...
