Dang, I typed a long reply and then Firefox suddenly quit on me!
Computers! Let me try again.
"Firebush", "Firecracker Shrub", "Hummingbird Bush" and "Mexican Firebush" are all common names for Hamelia patens.
"Firespike", "Scarlet Flame", Cardinal Guard" and "Mexican Firespike" are all common names for Thyrsacanthus tubaeformis.
So, you can see why plant names are so confusing.
I think the Hummingbird Bush may do fine planted near your Cersis, it grows to @ 5 feet tall. My next door neighbor has a large shrub right up next to the side of his house and when it rains that part of the yard is flooded and very slow to drain. We've lived next door to him for four years and I haven't noticed any issues with that Hamelia patens shrub!
I have Firespike/Scarlet Flame growing in full sun as well as some in a little shade in a well draining area of the yard. Just be aware that it grows 6 to 8 feet tall.
I don't know how cold your winters get, whether you ever have frost or freezes but being as they are small plugs, my suggestion would be to pot them up and hold them until spring before planting them out.
Hopefully others will pop in with some suggestions.