I'm still finding my way with flashes, but in theory any non-ttl flash should work on your Pentax and Canon SLRs ? I'm sure someone with a bit more experience will be along in a bit.
What do you want to use it for ? If just a party flash then it may be cheaper to get a body with a pop up flash - the Pentax ZX5/MZ5 is cheap and will take your K mount lenses.
Otherwise if you want to be able to bounce it, flashes tend to be a bit bigger