1) Can they be kept with severums and uarus?
2) How aggressive are they?
3) How big do they get?
4) Are they the same thing as red-headed cichlids?
5) Do all cichlids of this species look like this, or are these better-looking specimens than most?
These are Vieja species (probably melanura as the title of the video states), often called "red head cichlids".
They can be kept with those you mention, although similar shaped fish often can be competitors, and be aggressive. Severums or Uaru, which come from S America prefer softer water, these Vieja are from southern Mexico and northern Central America where water is generally mineral rich and hard.
In the video they are in brooding/spawning colors so their coloration in more intense than normal, although not excessively so.
They get rather large, up 15" so a large tank is needed, something over 150 gallons just for just a pair.
For a cichlid community tank, something around 300 gallons would be appropriate.
Below is a juvenile.
Depending on ancestral location catch point their coloration can vary a lot, some more dark with yellow, some from another river or lake more red.
I believe in a 300 gal you will "probably" be fine.
In a tank "half" that size I might be less optimistic, because Central American (Mexican) Cichlids tend to be much more territorial than their South American cousins, and any round, laterally compressed, similar shaped fish would be considered competition for space, breeding area, and food.
A 300 should have enough space.
I find Uaru very low on the aggression scale (maybe 2) Severum 4, any Vieja somewhere 7-9, but again, it all depends on tank size, and alpha status (breeding aside) Central Americian cichlids have an entirely different general aggression level than South Americans.
In a 300 it could be a pussy cat, in a 125, a terror.