Calculate symbol rate for QAM modulation
There is no inherent symbol rate for any modulation type.  64 QAM symbols transmit 6 bits per symbol, and 256 QAM symbols transmit 8 bits per symbol.  If the bit rate were to stay constant, then the 256 QAM symbol rate would be 6/8 of the 64 QAM symbol rate, but it appears that that is not the case (assuming your numbers are correct).  That may be because they are using more forward error correction (FEC) to mitigate the higher power requirements of 256 QAM.

Given that the bit rates are not the same, we can't guess what the 128 QAM symbol rate would be.

The general equation for a given modulation type and its symbol rate is:
$symbols/s = \frac{bits/s}{bits/symbol}$

Determining the bits per symbol is usually pretty easy.  It's log2 of the number of possible symbol points.  Thus, the bits per symbol of 256QAM (which has 256 possible points) is 8.

The trick is finding the bits per second.  Wireless transmitters often have the following blocks:

Data source => Compression => Scrambling => FEC Encoding => Pulse Shaping => Modulation

To know the final bit rate you need to know the bit rate of the data source, how much the compression reduces the data rate (if there is compression), and how much the FEC encoding increases the data rate.