From what i understand con's, especially a mated pair would work with bigger cichlids like O's, GT's, JD's, Severums, Texas cichlids, etc but i would avoid mixing the real nasty ones like the Jag, RD, RT, Midas, FH, Dovii, Umbee, etc. Like Fnesr said it is sometimes the luck of the draw you can still play the odds and odds are those guys i just listed will dominate the tank and beat up or kill the first ones mentioned.
I usually try to mix by general temperment and then size. Con's might be more aggressive than an O or a JD but they have the size to offset that. A con will likely not be as aggressive as a Jag or RD and also they have the size to really damage or kill the con. Of course there are always exceptions to the rule and it's your choice to try different mixes but then don't be surprised or angry if your Jag eventually kills the convict.
Why not choose from the mild to moderate agression range of cichlids like the first group i listed? Your chances are better that you'll have compatible species there as long as you don't overstock. Of course there are no guarantees there either but your odds are better.
What are the dimensions of this 300 gallon tank BTW? I'm just curious and jealous.