There are two main methods of giving players more than 4 races to choose from:
- Display a race selection dialog to players when the game starts and spawn starting units depending on which race a player selects in the dialog.
- Use the handicap setting to extend the race choices in the game lobby. This method is used by the Custom Race System.
The first can support any number of races, although beyond a certain number you need to implement a multiple-page dialog. Not an issue in your case since you only want 5 races. The problem with this approach is that not all players start playing at the exact same time depending on how long it takes them to select a race. Also, in-game dialogs are not that great in terms of UI design, I don't really feel that they belong in a custom melee.
The second approach can support up to 24 races, again not a problem in your case since you only need 5. The disadvantage of this approach is that players who play for the first time don't really know which handicap corresponds to which race - in your case, you can just make all handicaps below 100% result in the Naga race, which is simple enough that you can explain it in the map description.