Many tile editors will have GBA options as well, though specifically for the GBA I like
To swap/edit tiles you need to find where the sprites are located. It might just be a matter of scanning through with a tile editor and seeing it pop up, it might involve something more in depth http://www.romhacking.net/documents/361/
and it might be somewhere in the middle and see you have to play with some decompression tools (compression on the GBA is not half the issue it was for some playing in older systems), I cover a few compression tools in http://www.romhacking.net/forum/index.php?topic=14708.0
, however most would probably use http://www.romhacking.net/utilities/973/
for tile editing and any GBA/DS decompression tool really, GBAcrusher ( http://members.iinet.net.au/~freeaxs/gbacomp/#GBA%20Crusher
) is good stuff for compression if your decompression tools do not otherwise support it.
Adding new monsters is a more tricky prospect. You would need to figure out how the game assigns monsters, figure out if you can add more (if it numbers them with an 8 bit value and you already have 256 of them then you will have a seriously hard time making it to 257). Changing and existing monster is a less daunting prospect -- here you figure out the location of the stats, change it, change where the monster appears on the map/dungeon/arena/whatever, change where the monster entry you just cannibalised shows up lest the new monster appear there and you are done. Theoretically adding a new monster might not be much harder than that but you have to approach these things on a game by game basis. Adding whole new attacks on top of that is plenty doable for an accomplished hacker but by no means a trivial process. Adding some kind of new damage class or effect (is reflect in all of them?) is harder still.
As the NES/SNES entries are so well known you might gain some insight from data known for them, and the GBA FF titles are hardly new to hacking either, but do not count on it happening for everything as it was not straight emulation.