Actually, severum get big so it probably isn't a good tank mate for your angels. If you got another severum and formed a pair they would definitely kill the angels. If you keep him as a single specimen with the angels, and the angels decide to pair, then the angels will probably kill the severum. Personally I would just keep the one pair of angels in a 37. If you want other fish I would trade in the severum for something more compatible with your angels, such as a pleco that stays at a small size (like a bristlenose). You could also consider a small group of a 2-3" fish such as corydoras or rainbowfish.