1) to change font and size, ect..., first you must highlight the sentence you want to change and leave it highlighted and then change it to what you want by choosing the color and that is that.
2)a mizutamatchi can evolve to a young mametchi, obotchi,patopatotchi, piroriroritchi, hinotamatchi, nikatchi, and hashitamatchi.
3)yes, the codes are like what tamacool listed above, to enter them, go to the game icon and go to shop and stay at the place with the guy standing with the sigh that say shop and push the 'A' button several times quickly until the shop owner face changes and then enter the code according to the buttons.
4)it is because they are all souvineer, the souvineer are all for looking and collecting, you cannot play with them.
5)you cant play with the souvineer.
hope i helped
sincerely,Tamapal3000