Nice bike! Keep your 26" wheels. Drop down to 24" if you must but 20" in the DH Comp frame will be terrible. Cromotor sounds like a good choice if you are happy to have only a singlespeed gear at the back for pedalling.
According to ebikes.ca, with 26" wheels on flat ground with 120kg total weight you will need:
850 watts to maintain 45 km/h
1150 watts to maintain 50 km/h
1500 watts to maintain 55 km/h
Of course, accelerating requires much more power, but for only short periods. But, it's worth thinking about what speed you really want to maintain because it will have a big affect on the amount of batteries you'll need to carry.
Lets assume you want to stick with a 50 km/h speed and only recharge at home. Your 20 km commute will take 24 minutes at 50 km/h. 24 minutes at 1150 watts is 460 Watt-hours (Wh). Let's add 20% margin for accelerating or small hills (552 Wh). Now, discharging lipo below 20% capacity will shorten battery life so add that margin to find the rated battery power you'll want to carry (662 Wh). The batteries you selected have a nominal capacity of 666 Wh, so I think you've chosen well for your situation.
However, it's always nice to have a larger than necessary battery for the unexpected extra trip or if you miss the chance to do a full charge one day. Other options included keeping an extra charger at work, or having three spare 6s packs in a backpack to use as an emergency 18s1p pack. For example, carrying 3 of these
would only weigh 1.8kg and give you another 266 Wh of power, or 14 minutes extra riding at 50 km/h.