Did not work out. Kupang is bagged and about to take back to the fish shop.
I noticed today the RG was missing part of his tail. Further, the Kupang has essentially stalked him all day as I watched. When I saw him attempt to jump the RG from behind multiple times in quick succession I decided to pull the plug.
I had noticed they were not getting a long for about a week, but seemed to be chasing only until today. RG had ceased hiding and was standing his ground. Whenever the Kupang charged, the RG would flare his teeth and take a defensive stance. Kupang would back down then try again shortly later.
That said I've seen this duo working fine in bigger tanks. I think in my case, a 20 long just isn't enough space for them to cohabitate happily. Seems like sites are saying this is the min tank size for the Kupang, but some recommend 50 gallons. Watching it move, I can see why. It's a much more active swimmer than the clowns or RG. I think it will be happier in a larger tank where it can have sufficient territory. I think the "semi-aggressive" title that LiveAquaria assigns it is valid for this one. I was too quick to consider the Springeri and Kupang as equally peaceful. Had my RG been fully grown instead of a juvenile maybe that would have made a difference.
I plan to move up to around 120 in year or two. When I do that I may try one again. That is unless I decide to roll the dice on getting a "true" springeri.