Ive heard plants with bichirs and oscars get up rooted very easily, and the pleco will help with moving plants around. You have lots of bio load with the oscar and pleco, but you have lots of filtration so i suppose thats ok, what bichir were you thinking of getting? There is many species of bichir so you will need to expand on that. I would suggest these bichirs for the 75 gallon,
P. palmas palmas ('palmas' bichir)- 13"
(subspecies)P. palmas polli ('marbled bichir)- 14"
(subspecies) P. palmas buettikoferi ('buettikoferi' bichir)- 13"
P. delhezi ('armoured' or 'delhez's' bichir)- 14"
P. senegalus senegalus ('Senegal' or 'Cuiver's' bichir)- 12"
(possible subspecies)P. senegalus meridionalis- 12"+
There is, Also the possibility of a third member of the Polypterus senegalus
group that is found in the Nile River
P. retropinnis ('retropinnis' bichir)- 14"
P .mokelembembe
E. calabaricus ('Ropefish')- 15"-30" (Closer to 15''-20'' in captivity)Best kept two or more
I would also recommend at least 2 of bichirs because they all feel safer with another bichir with them