Many Vieja can be a tad more aggressive than severums (melanurum (AKA synspillum)are usually considered less so), but still become territorial with any other similar body shaped fish in limited space. If you tank is in the 100+ to 200 gallon range or larger, you may be OK.
Vieja also usually grow at a much faster rate, and as they grow (in any size tank) you will need to up your water changes accordingly.
Vieja are a hard water species, severums are generally a soft water species.
If it were me, I wouldn't combine them and I'd keep the type that fits my type tap water, if your tap water is neutral, it may not matter
In hard water severums often get HLLE scars as they age , in soft water, melanurum may show similar maladies.