Let me give this a shot.
1.) If you mean something like, where you talk to a character at one point and you choose your class, then yes, through Show Choices, Labels, and Switches. If you mean being to freely change your class throughout the course of the game, then that's possible too, using Switches/Variables.
2.) On the main RM2k3 screen, click the icon to the left of the music note (it looks like a square with arrows going around it). Click "ChipSet" and the click Import. Pick the Chipset you want to Import and click Open. If it brings up an error, then the chipset is either the wrong size or it's the wrong color value (find out how to fix that
here). Then, go into the database (the icon to the right of the one you clicked). Click the tab that says "Tileset". Then, click "Array Size" and increase it by one. Click "Tileset File" and change it to the one you imported. Now, you have to go through and set the Passability, which determines whether the player/events can walk on it or over it. An "O" means that you can walk over it, an "X" means you can't, and a star means that if you walk on it, it'll appear over you. You can also determine the Directional Pass, which determines specific directions that a player/event can/can't walk over it.