They should, in theory, stack, (one being a resistance, the other being reduction) but you're not going to get 10/+1 DR from a slashing attack.
That said, the best will always apply -- the "5/- slashing" will still apply even if attacked with a +1 or better weapon, which would negate the "soak 5/+1". With this combo, you essentially have "5/+1 AND (bludgeoning OR piercing)".