I hade my percula clown in first (about 1 month) and then i added the 3 sebae clowns
The percula is about 3" long and the sebae's are around 1" to 1.5" long.
If your sebae's are the same as mine you should be able to add a percula the size of mine. I would let your sebae's have the tank at least a month before you put in a bigger percula. That way they know who's boss when the bigger one comes in