If gaming is the only thing you do then it isn't the best monitor fit for purpose, it is however a lot better than the 2407WFP when it comes to gaming (and everything else).
I assume you're going to use two 2407's and one u2412m? I doubt combining a new monitor with two that have degraded over the years would be a good idea but resolution wise it fits. Both are adjustable in height and have thin bezel as well so lining them up should be easy.
So to answer your questions:
Is it good for gaming? It's very decent and a lot better for it than your current monitors (among the best IPS-panels for it!).
Can you use it along side a Dell 2407WFP? Depends on if you have the connections for it and either NVIDIA surround or AMD Eyefinity. Also, you need a total of 3 monitors of with the same resolution (these Dells do have the same res).