Regarding this question of yours, yes it's okay to use your existing antenna for Main and AUX of RUTX14 and connect the standard antenna on the remaining AUX antennas. But I am not sure if you will be receiving the same speed throughput in this setup when using the 4x4 MIMO antenna since when you are using a 4x4 MIMO Antenna it will be a full outdoor antenna with the same mounting as when you are going to use your existing 2x2 MIMO Antenna it will be outdoor and the rest will be stick antenna's that will be mounted directly with RUTX14 there are different antenna mountings so it may affect the device performance. For this reason that is why it would be best to actually test it on the live scenario. Because as per standard LTE CAT 12 uses 4x4 MIMO in order to reach the ideal speeds.
I hope his help