If you're going to stock an enclosed water, there is no harm in feeding them. This way you can increase the numbers with less impact on invertebrate life. Problem is if you stock a lot of fish you are opening yourself to thieves, sorry I mean poachers and hungry wildlife.
On the triploid front, the triploid process is meant to only be 90% effective, I think it's less. The environment agency insist on triploid stocking because of interbreeding with wild stocks.
Once you have registered your water you are free to get your said species from anywhere, it is now down to the seller to do the