Sp.reds are very prone to making hybrids.
If you must absolutley pick a thrid variant, pick a Kasabae (rainbow type) in stead of a sp.red or other southern moorii.
Sounds like a beautiful tank. If you got the 6ft wall space, the 180 gallon will be a much better fit for (3) variants.
Duboisi will dominate breeding and produce more fry. The other 2 will breed almost nothing at all. Your breeding in this tank will be very minimal. Stripping will almost be a must to have any success.
Make sure your filtration is in good order and maintain bi-weekly/weekly water changes of 30-75%
Best of luck!