So could i get like 2 of the same species like 2 oscar or 2 red devils in a 75 (sorry im new to this) i would have 300+gps filter and do a 10% or more water chage a week
No, the amount of filtration you have doesn't change the fact that the 75 will still be only a 75, not a large enough footprint for a pair. Conspecific aggression can be really bad in these species, and red devils typically live up to their name.
Do either a single oscar, midevil (most red devils or midas' are hybrids of the two), trimac, OR a pair of perhaps nicaraguense, or jack dempseys (not my personal preference), or a small geophagus colony (red heads??)
Also in regards to your water changes, you'll need to be doing at LEAST 25-30%, preferably 50%+ per week. Bigger fish = bigger mess. Also with your filters you generally want 10x turnover per hour. so 75x10= 750 GPH turnover. I'd add an AC110 if I were you.