Advice on tankmates for 2 oscars

hello i have 2 albino tiger oscars that i got when they were 1" 1/2 or so in size and are now about 5". currently they are in a 55g with 1 pleco who is similar in size.
In about 1 month i plan to be moving them into a 90g along with the pleco and was wondering ,if any ,which tankmates could be suitable. ( i know it greatly depends on the temperment of the fish)
i have had them with 2 clown loaches but the loaches didnt make it 3 weeks before they were dead (of stress i believe). the 2 oscars whose sex i am unsure of seem to get along since the day i brought them home except the odd shoving matches.
the tank is roughly 48"x20"x22" and would like to find 1 or maybe 2 more fish to go along with them. i do really like jack dempseys and hear it is possible for them to live alongside oscars, just not sure if 90g is too small for that to happen.

i see, so how would 1 pleco and 1 oscar do in 90g?
the impression i was under is that the 90g would suffice the 2 oscars and the pleco (pleco temporarily due to mature size).
i am new at this so i had probably picked the more expensive side to start on hehe.
previous i have only kept a 55g with a dempsey and 2 convicts.
when i moved cross country my friend adopted my fish and i took the empty tank with me, thats when i thought oscars would be ok. i found out i was wrong with 2 in 55g and that 75g+ should house them fine.
now i find out i need 110g so i would be willing to give up one oscar to save me $500 +.

tyvm for the advice.
i also have one other quick question, in the 90g it is a reef ready tank but i will be temp. plugging the 2 holes in the floor of the tank with a screen, bulkhead and ballvalve and using it just for waterchanges. i would like to know which filtration would work best w/o using sump (or any type which requires plumbing through the 2 holes).basically HOB style or submersible.
i know the sump setup would work better but its not an option for me atm. i just need to know which of the previous filters would work best with a 90g housing 2 oscars and 1 pleco(leaning toward giving away common pleco for smaller type).