Oscars tend to be fairly non-aggressive compared to the others....
Problem with your stocking plans is that all those fish can vary drastically in personality and aggression level. It may work out, more likely it won't. It may end bad for your oscars and sev especially if your other cichlids form a pair or become overly aggressive. I see a lot of oscars with their eyes bit out trying to mix them with extremely aggressive cichlids like red devils/midas and jags in an overstocked situation because people think oscars are aggressive, usually they're not. I've seen severums torn to shreds, because honestly severums tend to be pretty peaceful a lot of the time.
The other problem is you can conceivably end up with breeding going on even if you only pick males or females of a species. Though it seems you want male and female cons. Not real good idea if you want to keep the peace in this type of setup.
Jags,midas, convicts and firemouths can all breed together. Convicts especially will breed with almost anything so it wouldn't be hard for you to end up with some kind of breeding pair, and when these types of cichlids breed they aren't nice.
Jags and midas can be extremely aggressive, so if either start breeding you will have problems. Convicts aren't much better, the only plus is that they're smaller than the other two.
Say you don't end up with a pair of something, considering the size of the individuals combined the tank is really too small.
I guarantee you will NOT be able to keep plants alive with these fish, so planting won't do anything for you, lol. My red devil builds a 5 inch high mound sometimes in the sand, if plants were in there they'd constantly be uprooted. You can try some plants on wood but they'd still likely be ripped off.
Your pleco is likely to be picked on, at the worst killed once the others are full grown.
Now - 400-500g? That's more like it. Still can't guarantee a peaceful mix though, just not possible with the indivudal personalities of cichlids.
That is the good and bad side of SA cichlids. They are great because they have their own personalities, however it's nearly impossible to advise what mixes will work and what won't.
If you are willing to remove breeding pairs as they form it will be much more likely to work...