To answer your question about overstocking, you are not at all overstocked at the moment. You have a fairly light bioload. When the severum is fully mature it will be a bit large for your tank and produce quite a bit of waste, though a 40 breeder is not necessarily inadequate for their size due to the footprint. I would not however suggest keeping it in that size tank with other fish once mature since you may have trouble keeping up on maintenance, and he would be better off in a larger tank really.
Your bristlenose pleco will not get 2' long, that species only grows to about 6" max, so you're fine.
But I think people are correct that your tank was never cycled. A cycled tank will have 0 nitrite and ammonia, so if your ammonia and nitrites have always been present it was never cycled. I'm not sure if I missed the information, or you never posted it, but exactly how long has the tank been set up WITH fish in it? Cycling a tank generally takes 1-2 months after an ammonia source has been added, but it can take a bit longer for some people due to various factors that effect the growth of the bacteria.
You could add another filter (do not remove the one that's currently on there since it is currently establishing bacterial colonies - then you would have to start all over again with your cycle), and it's always good to have redundant filtration, but the biowheel you have on there should be sufficient for the bioload you currently have.
One thing that may have prolonged your cycle is either ammonia or nitrites becoming too high and you not noticing. When ammonia or nitrite gets to high levels it actually impedes the growth of the bacterial colonies, causing it to take longer for you to cycle or stalling your cycle altogether. So you will need to watch your ammonia and nitrite every single day and do water changes to keep it around .25-.25 since you have fish present.
Also, adding Prime to detoxify the ammonia and nitrite will actually aid your bacterial colonies since high levels of ammonia and nitrite also damage them physiologically, not just the fish. But the ammonia and nitrite, even after Prime is added and has detoxified it, will still be available for them to consume so they can grow adequately.