Martin,
What rms signal? You have not shown any code with any rms calculations.
1. RMS, by definition, is an average over some number of samples. So any rms calculation will lag behind the data going into it. This will affect the timing of any trigger.
2. While the Trigger and Gate.vi seems to have several triggering options, it does not appear to have a trigger index output. If you wish to trigger on some combination of different signals, especially when there are delays between them, the index is very useful.
3. You can convert most Express VIs to standard VIs by right clicking on the fornt panel. Then you can open the block diagram and make any changes you need.
4. While Express VIs are OK for quick and easy operations, as soon as things start to get more complicated, their limitations become apparent. You will probably need to write your own code for the RMS trigger.
Lynn