Garmin Map management

G'Day all, Happy Australia Day,

I'm currently finding my feet with a nuvi 760, not new to gps but new to this unit, so please bear with me.
(previously had magellan and recently, briefly tom tom)

I am wondering how to manage maps. I have a full back up of unit as out of box untouched, so can always get back to square one.

I got shonky maps, and another one with three all rolled into one, (shonky, tracks4 australia I think and another)

but when installed they look good off road but I think they impair the on road maps, is this so?

If... for example,... I have latest garmin map that has say a new freeway or something, then install shonky, does that go over top of latest map and eliminate the new freeway?

And if so how do you take shonky out.... do you need to delete whole thing n reload a back up?

Wondering how others manage the maps...??

I dont go off road a great deal, but dont want to spoil my latest map from garmin.... can I keep shonky on an sd card and insert when I go off road, then take out when I go back on highway, if so will it then revert to my latest garmin or do I need to do a restore from my pc?

I'm very impressed with this unit, but need some help to get the best out of the maps available.

As usual any help much appreciated.

Rgds

Ron