Hey fishy sorry i didnt answer.. completely lost this thread

Its good that you went with 6700/10000 you should get a nice color output from that.
Some harder to grow high light plants you can try is. HC, L. aromatica (will get nice purple color with high light), blyxa japonica (will get orange/pinkish with high light), red e. telenus (spelling), ludwigia repens x arcuata (gets really nice color with high light), downoi, different toninas (pricey plant)
Basically as you can see i didnt really list a "harder" plant to keep, but a plant thats harder to get in perfect color.