To add a model optimized for SFC 1, go into Assets/specs and open up shiplist.txt with Microsoft Excel. This is a list of all ships in the game and their attributes. If you scroll to the right you will see a column where the directory path for each ship is listed. You need to change the directory path to the new path for the ship that you are adding. It looks something like this: assets/models/fca/fca.mod
Please note that the slashes are the opposite way as shown in your browser address bar. As far as I know, the directory listed can be anything within the main SFC Directory. Also, in SFC 1 you can only replace ship model directory listings, you cannot add additional classes as you can in SFC 2, OP, and III.
"Optimized for SFC 1" means that there are no light maps mapped to the model. You can change an SFC2, OP, and III model to work in SFC 1 by importing the model into your modelling program and removing the light map material, and then exporting the ship back. Milkshape can do this, however, It cannot do the break models. Remember break models need to have the light maps removed if you are using them.