I had a Senegal bichir attempt to eat a pictus cat once, thought both fish were going to die. Bichir eventually coughed up the pictus and lived. But I wouldn't recommend the two together, bichirs tend to eat elongated fish.
On the cichlids; if you have water with a pH of 6.5 that's not really ideal for firemouths or other central americans. There are many ways to raise your pH a bit though, I would recommend a bag of crushed coral in your filter. Won't be a dramatic affect but will help add more mineral content to water and should bring up pH a bit.
The severum would do fine in the lower pH but personally I like to keep CA fish with CA fish and SA with SA, due to the differences in water preference. Though, both fish have been in the hobby for a long time and will be adaptable if you get captive bred specimens (mostly what's available)
If it were my tank I would pick either the severum or the firemouths and build the stock around them. Severum, perhaps blue acara, some large bodied tetras, your pictus cats, for a SA theme. Or, group of 6ish firemouths with some livebearers like swordtails, which they live with in wild and makes a nice display.
Another possibility, which the purist in me finds hard to recommend, would be to keep a severum with your firemouth and add your pictus cats, which seems to be what you want to do. This might actually work fine, but mixing different species of cichlids especially from different continents is a balancing act, and it's going to be up to your fish whether they play nice or fight each other. I'd say it's 50/50. Better odds of working out if you add one more smaller cichlid like a blue acara. If you attempt this, have another tank running on standby incase you need to remove a trouble maker.
