Can I use the same dongle for monthly leases as well as having some additonal runs (from 50 run packages) or do I need sepearate dongles for that?

Meaning, if I have a 50 run license, will installing a monthly lease on that dongle wipe them out?

No, you need two separate keys. If we put both limits on the same key, the software will stop when either limit is reached. This isn't what you want.