MoMule, If your just looking to add a few Koi for fun I would say go for it, if their large enough the trout won't bother them and a few shouldn't affect the pond.
If you go with single sex that would eliminate problems down the road, and half the fun of having a pond is experimenting.
This place ships tiger trout, maybe if you had a cage to raise the fingerlings might be worth checking into.
http://www.coldspringstroutfarm.com/pond_stocking.htm
