I can appreciate the appeal of a mixed flock, but if you are wanting rare breeds to help the breed, what would be the point? Would you have loads of tups so you could breed each pure? That's quite a cost and management overhead...

Or if the sheep are just lawnmowers / fleece sheep, then yes by all means, buying a few wethers of a few different breeds will help the breeders, and no reason to not have a selection of breeds appropriate to the terrain.
Herdwicks originate from the Lake District, so although they are used to a lot of rainfall, their feet are maybe a bit more suited to hard rocky slopes than unrelenting sogginess. Most of the northern wet moorland type ground seems to carry mostly Swaledales, which are fabulous, adaptable sheep - but are not rare. Similarly Scottish and Northern England type Blackfaces.
There are other listed rare breeds which would maybe suit the soggy ground, but not all sheep's feet cope well with being wet all the time.
Good feet for wet ground, my mind leaps to Exmoor Horns. They are not a listed rare breed (yet

) but they are not widespread outside their home area, so a more northerly flock would give some protection from losing the breed to a localised problem in the SW.
Other contenders which are listed rare breeds might include Derbyshire Gritstone, possibly Whitefaced Woodland : there are folks on here have those, so hopefully can comment about the feet and their suitability or otherwise to the ground. I had two local friends with Llanwenogs in North Cumbria, so their feet must suit wet ground.
If you wanted to go very rare / primitive, I think Boreray or North Ronalsday might be worth a look. Manx Loaghtan too. Hebrideans seem to be happy pretty much anywhere

, [member=4333]Fleecewife[/member] can tell you more about them. (Not rare though, same with Shetlands.)
Would you make use of the fleeces? Handspinners

are not so keen on Herdwick, too harsh! (Although dyed Herdwick is awesome for weaving for rugs, bags, etc.) Exmoor Horn isn't exactly sought after, either. Boreray is rare so has a value, although it's not actually very nice to spin or use. Derbyshire Gritstone and Whitefaced Woodland can both be lovely for handspinning and dyeing, Llanwenog is fabulous, North Ronaldsay is a double fleece so takes a lot of work but is interesting because of the colours...