Oh now I really must protest!

My choccie lab is the sweetest floppy-lipped dog you can meet and trust me, he is smart, gentle lovely and gorgeous!
A bit of a softy, but has a wicked sense of humour (ask my trainer!)
To pick up on another comment, he is indeed bouncy, however as a young pup he happily ran around with a sheltie and he never played roughly with her or squashed her - he seemed to know she was not a lab and so would not appreciate 'wrestling'. Her owners had another older sheltie that didn't like to play. If Cadbury went up to it, it would give a quiet growl and Cadbury would wander off and leave it be.
Its all about socialisation. If you could see Cadbury with a small puppy you would realise how gentle a lab can be. The other week he met a staffie puppy, only about 10 weeks but full of beans, it wanted to meet Cadbury and he laid down so the puppy could sniff his nose, then he rolled on his side and rubbed his head against the pup and the pup rubbed back and then he started bouncing on Cadbury's head, and Cadbury let it and just laid there.
The only thing with a lab is they are so so stubborn! Sometimes people think this makes them stupid, but that's not the case, they just need to know that what they are doing is worthwhile.
Well, I've stated my case for a lab - tbh, I think any dog breed from a good breeder, with socilisation and training should be able to mix with any other dog breed, and I think shelties are fully capable of looking after themselves with big dogs