Nothing wrong with your temp as those fish will tolerate anything between 25 - 28C. Most people have their tanks at 25 - 26C but mine sit at 22 -24C and I mind my fishes quite happy.
Are your 'plecs' actually plecostomus (Hypostomus plecostomus), or the red sailfin plec (Petrygoplichthys gibbiceps) or actually bristlenoses (Ancistrus)? There is a big difference in adult sizing in those species. Bristlenoses only grow to about 8 - 10cm, the red sailfin to around 30 - 40cm and the common plec will grow over 60cm.
Plecs continually graze off algae on plants and rocks and a new tank will not have any for them to eat, hence then preferring a more mature set-up. They will cope with the pellets I am sure.
If you have a problem with water conditions I would expect it to show first in the clowns as they are very sensitive to water conditions.
If the fish are still acting normally and feeding well, I would not worry about their colouration too much at this point.
Plecs are nocturnal so will be more active at night. Besides, as fish go, they don't move much anyway