You can not use 6v6s together with 6l6s, different voltage requirements.
Or even with different power supplies to different tubes it does not make sense tone wise.
4 6V6s will give you about 40 – 50 watt and it will sound nice, will brake up earlier than 6L6s.

From what Nik has advised me, 6V6 tubes are ok in the D amp clones as long as the bias is set correctly.
You will need to be careful which brand and type are used, and this is because the construction of the traditional 6V6 will not take the plate voltage in this amp.
Both the JJ and TAD 6V6 STR will be fine as these two different are built with a highr spec to handle both plate & screen voltage.
Do a web search for JOHN TEMPLETON`S 6V6 tube review and you will see that these tubes can take the higher voltages.
Apparently the JJ is a little more aggressive in tone, while the TAD is smoother and a little warmer.. more tradional 6V6 type tione.
Nik did say biasing 6V6 tubes in the OTS shouldn`t be a problem, although a simple resistor can be added in the circuit to get the bias down to the 20 -  24 mV range if the bias pot doesn`t quite get down low enough.
My OTS 100 in 50 watt triode mode is very smooth and probably a bit more 6V6 than 6L6 in tone because of the nature of triode mode on the power tubes.
In saying that, a quad of 6V6 with half power triode option would be interesting in one of Niks D clones.
Cheers...