This is not very common with Raja and most Indian music composers.
Mostly, the mixed chorus has been used by Indian music composers for patriotic songs. Raja has used them for different situations, though only on few occasions. However, this is more prevalent than Raja using traditional male chorus.
Andhi Mazhai Megam in Nayakan (Tamil 1987). Some parts of the pallavi are sang by the female chorus. Observe the charanam closely, when the male singer sings, the voice is backed by the female chorus singers and vice versa. The transition between the pcharanam and the pallavi is executed with mixed chorus. The second interlude has some parts executed one after by the other by the female and then by the male chorus.
Let's hear Andhi Mazhai...
Raakamma Kaiya Thattu from Dhalapathy (Tamil 1991) One of the most famous tracks of Raja, he has used mixed chorus effectively in this track. All the ‘jaga jaga…’ has some parts sang by the male chorus and partly female chorus. Perhaps, the most famous mixed chorus lines in Tamil can be ‘jaangu jakku…’ sang in this track. It’s all over the place in this song. The third interlude in this song is arranged as an abheri set thevaram sang by the female chorus. The final pallavi is sang as the regular light music piece with the female chorus continuing the abheri humming in the background.
Adi Raaku Muthu Raaku from Ejamaan (Tamil 1993). Parts of the pallavi are sang by the female chorus. In the charanam, some lines are sang by the male and then by the female chorus. Also, the chorus is used to transition from the charanam to the pallavi.
Maadathile from Veera (Tamil 1994) (throughout the track). The mixed chorus constantly backs the main singers throughout the song. The track starts off with them. The pallavi has the mixed chorus backing SPB taking turns with him. The first interlude has the mixed chorus taking over. SPB switches to the background with the mixed chorus in the foreground. The mixed chorus is used for transition between charanam1 and pallavi2. Interlude 2 has many parts executed by the mixed chorus. The second charanam is arranged similar to the first.
Let's hear Maadathile...
Isaiyil Thodanguthamma from Hey Ram (Male choir singing jathi) (Tamil 2000). This is a very different track where mixed chorus is used in a trance song! Some parts of the pallavi has the mixed chorus not only singing some lines, but also the ‘than dhim dhim…”. The charanam to pallavi transition is also done using the female chorus during the first such transition. During the second occasion, the main singer (Ajoyji) sings swarams as part of the transition.
Chellathathee from Manassinakkare (Malayalam 2003). Parts of the pallavi are sang by the mixed chorus. The first charanam is arranged a bit differently. Mostly, the singer is backed by the female chorus. The second charanam has the main singer backed by the male chorus.
Let's hear some mixed choir in Chellathathee...