There have been crossbreds bred for working purposes for centuries - lurchers for example, a whippet x bedlington produces a pretty reliable type, collie to greyhound, collie x greyhound back to greyhound... and all the other interesting mixes, have produced reliable and reasonably predictable types for a long time now.
But trying to create a new breed is another matter.
I would look at previous attempts and see how they went - historically, inventors of new breeds were one man operations, who retained control over all their stock and culled hard (yes the way you think I mean that) so they achieved their goal reasonably (ie, in their lifetime) quickly.
That just isn't possible these days, breed creators today do not have oodles of cash and a country pile to use, they can't and shouldn't keep 10s of dogs in kennels whilst they work out what breeds true and what doesn't.
so other people get involved and that is where it goes tits up - people disagree, people go off at a tangent, goals shift and crucially... all the 'not quite right' puppies, what happens to them?
Look at:
Rodinglea Scruffies (Beardie mixes)
Utonagan/Northern Inuit/Tamaskan - a shit show and dogs riddled with cancer dying very young.
King Shepherds
Panda Shepherds
Plummer Terriers (a more successful example but theres very few around now)
Silken Windhounds (very very few of these around)
Cockerpoos (so massively variable and are not the easy pet dogs people think they are on the whole)
Labradoodles (ditto)
Australian Cobber Dogs (ditto)
Just a few examples off the top of my head where people have tried, some with reasonable intentions, to create a breed and it isn't the success they had hoped for in a variety of ways.
IS there really a demand for a Tofty Sheepdog?
Who is this demand coming from? Surely those people would already own something like a Rough Collie or close to that.
What does the Tofty Sheepdog have that another pastoral breed does not?
To breed, you need people demanding that breed, you need a list of definite puppy buyers before you even think about putting bitch to dog.
This is a vanity project for the inventor of the breed, I am not saying it is impossible to produce some nice pet dogs - it isn't, if the temperament and health of the parents and grandparents is good, if you vet owners properly, if you/the creator of the breed can absolutely supply lifetime back up for these puppies should their new owners situations change... The world DOES in fact need steady, sensible, bombproof pet dogs.
But to create those, I don't think I'd start with Rough Collies any more than I'd start with mixing cockers and poodles!