I think it's not a good idea to remove the trim if it's glued to the Rockford speakers.
Even if you use the oven method to weaken the glue of the trim, there is the possibility the the cone will also come off. And this will damage the speaker, because the cone has to absolutely centric and assembled without any tension. I think it would be better to sell them and to buy another pair of speakers without a trim. In germany this is called euro norm speaker.

There are some reasons why it's better not to use a coax/triax:
1. The staging will be much to low.
2. If you use the stock tweeters in the doors and the coax/triax this will cause a high impedance in high frequencies, which will cause that the high tones are too quiet.
3. You also can have problems to use the top part of the speaker bracket back, which will cause that the low end of bass will be killed because there is a gap between the door board and the speaker bracket. In other words it's not sealed, which will cause interferences and a loss of bass from the doors.

Better solution:
Use a two way system system with an euro norm 6.5 chassis and a good crossover. Position the tweeters in the a pillow. Make shure that the crossover has a -6dB (or more) lowering for the tweeters, because the tweeters could be too loud if you position them in the a pillow.

This is an example where a SLK a pillow was modified this way:



Regards
Markus