On my OS 15GT, 33GT and 55cc engines I have formerly used four cell Nimh on all of them but currently all have been changed to six cell Nimh(6volt). This is not because there was a problem but only in the interest of compatability with what I keep in my personal stock all the time.
I do have a DLE 20 but it is still in the box and I am not sure what I may decide to use but probably the six cell agine. what I would not do and perhaps its just me ( yes my speculation) but I would not use any battery system or battery chemistry that exceeded the nominal maximum listed voltage. Also agine just me but I would not use any added complication of a battery chemistry that exceeded the nominal voltage and required the use of a voltage regulator. To me that is undesired complication.
For my stuff, the battery spark ignition gassers simplicity is my goal and that means two separate independent battery systems, one for ignition and one for flight control and using the same chemistry, voltage if not the same capacity. This very much simplifies charging considerations
John