Hey RD,Unless your flowerhorn is a short body, your tank is going to be too small for long term.
Thanks for posting pics of the tank!!
I would probably take the African cichlids back to the store if you can since it appears you will be using the 260 litre tank for your new FH.
A 10G seems a bit small for an adult though you could always try it.
Hey,You'll want a tank big enough to house him with at least 6 females. More rockwork would help too, you'll want spots fish can duck into if being chased.