Best RapidLED set for a 20L

Probably around what your canopy would provide, 5-6" or so.
I'd do a 50/50 mix of whites and royal blues. The Rapid LED 24 dimmable kit is exactly what I'd go with.
For heat sink, I'd ideally recommend 26-28" for your tank, but you'll be stuck with 24" if you want a drilled one from Rapid. Shouldn't make a huge difference however.

Looking at the solderless connectors, I see they're listed only for the XR-Es. However, I can't think why they wouldn't work with the XP series; I'd check with Rapid on that one.

Lastly, I'm willing to bet your PAR bulbs only have blue LEDs, and not royal blue. It's this shift in spectrum that makes the difference compared to the Mu135 you've got.

 

Along with the UV LEDs in the Mu135W fixture. I cannot tell you what a difference they do make in coloration. I have an Acan Lord colony that now has a yellow/green ring around its mouth on each polyp that developed after adding the UV LEDs. All my corals show more coloration, as they did under my Mu135W fixture compared to my CREE setup. They are hard to find/get and can be quite expensive, but IMO, well worth it.