Yes, you can mix transducers with the NI 4351 device.
In LabVIEW, there is an example program where you can select what type of transducer you have on each channel.
I've attached this VI below for LabVIEW 6.1. If you are using an earlier version of LabVIEW, you can get to the example by going to the 435X functions palette -> Examples -> NI435X Getting Started Multiple Transducers VI.
One thing to note when measuring multiple transducers is that the 435X devices can only have one gain setting for all the channels. When measuring only thermocouples, it is set to the highest gain setting for the highest accuracy. If you are also measuring a voltage that you cannot apply the high gain to, the accuracy of the thermocouple readings will go do