the similarities are there, but they're not hugely similar.
Mandos are mercs and bounty hunters. Klingons are not.
Klingons as you say, have an Empire. Mandos don't. although, they do live on several worlds.
Mandos are extremely adoptive and inclusive. Klingons aren't.
Mandos don't recognise much difference in the genders. Klingons, do.
the fact that both are warrior races and use a mix of high and low tech weapons and have their own code of honour is rather less than the differences.