Yes the SB-400 is the perfect size for a small camera.
Just a small caution - using the SB-400 you will not be able to set it to "Master" and control other speedlights. The P7X00's are not able to control other speedlights with their built-in flash but if one of the higher spec speedlights is fitted to the camera then you can control remote speedlights.
If you are not planning on controlling other remote speedlights then I think you will be happy with the SB-400.
Regards, Clive Liddell Pietermaritzburg South Africa