Hi,
  At places like the rspca can you only normally get adult dogs or can you get puppies too, also if you got a puppy would they know what it was crossed between?
                                                  
                                                  Thanks.

The RSPCA shelter near me normally has all age groups - including whole litters of pups at times :(  As to knowing what breeds they are or have been crossed with it will depend on location and circumstances of how they got the pups in the first place.

Better to enquire directly with your local dogs' home.  Regards, Teri :)