Certain sprites are hard-coded to use the sprites out of a certain box. Here's a list of which spritesets use which one:
Box #1: Cemet, Cent, Eggs1, Eggs2, Guest, Hell, Island, Kings, Maze, Moon, Plant, Priest, Ravil, Red, Sand, Stream, Toro, Weed.
Box #2: Almo1, Almo2, Ballos, Bllg, Curly, Dark, Dr, Frog, Heri, IronH, Miza, Omg, Press, Ravil, TwinD, X
Regu and Sym are always loaded, obviously.
Island is defined in the first box, yes, but I'm not 100% sure how that actor or its map works, really.
Ravil is actually used in both boxes, interestingly enough. However, the second box only defines it for the Igor Boss actor and the first box is used for Ravil and Enemy Igor (for Balcony).
Hmm... hope this helps a bit. It's a shame there's not an event flag that tells the program which box to use. Pixel didn't need to code that apparently, which is kinda sad because it would be useful as hell to mix two character sets together and see how Power Critters and Gaudi Armors complement each other.
Note that with a bit of npc.tbl hacking, it *is* possible to change which box a certain sprite takes its graphics from, so it's possible to achieve the above example to a degree. If you were to find all the actors from NpcMaze and change that flag so they use the second box, you could then put Weed in the first and Maze in the second for some added fun!
But aside from that, I guess that's one of those "Wait For Agora's Legends" features, or some such. ;P
[EDIT]: Egad! I swear, those last two posters jumped ahead of me! Ehehe, so Garde's answered most of this stuff already. Thanks and stuff!
And somehow, I never thought of that idea until now. I recall Crowned Witch's "Red Hell" stage now, which has hopping critters and Fire Whirrrs, mixed from two different tilesets. Hehe, very interesting idea, really. As long as the spaces don't overlap, it's very possible to add additional characters from a different tileset. Weed has a bunch of extra space toward the bottom that might come in useful, somehow, so maybe that example of mine is easier than I thought. Thanks Garde. ;P
And Atravir, your problem would actually be solved easily by changing the Ballos actors to use the first set instead of the second. Balrog is more of a common character than Ballos anyhow, and he usually appears in maps with enemies as well so it's good to keep him in the second set.
On second thought, are you using Ballos's first form (in NpcPriest)? 'Cause if so, he's already defined in the first box and the two can be used together already. Also, if you somehow need to use both Ballos forms + Balrog (or have some desire to use characters from more than two tilesets in a cutscene), there's a neat little method I devised for my ever-so-elusive mod. You can actually copy the room into a second map (with different actors defined), and if you don't use the fadein/fadeout commands while using <TRA####, the transition between maps will be instantaneous. I've found this one VERY useful myself. ;P