As Skeptic says, undersized alternators can be an issue. Oversized not so much. An alternator is controlled by the regulator so it will only put out enough to match demand until it hit's it's limit.

So, if you have a 120A alternator in an 80A system you'll be fine and have a 40A reserve, if you have a 60A alternator in a 80A system you will be draining your battery trying to support the load.

However, if you are going to run a 120A alternator you had best make sure your wiring and protection matches.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.