The stress coat and zyme coat stuff really doesn't need to be added with your 10-15% water changes (FWIW, you CAN change more that 10% if you like...whatever you choose, make it a routine and stick to it--this will keep the water somewhat consistent)
The only thing you really need to add to the fresh water, during a partial water change, is a chlorine/chloramine treatment. Or, if you really wanted to you could do the water changes with bottled water instead of tap water--you have to be careful here, though, as not all bottled water really is "natural spring water" a lot of it is just bottled tap water. We have a bottling plant here locally that pulls water from fire hydrants to put in their bottles
So that would still need treated for chlorine/chloramine.
(This just confirms that bottled water is unneccesary and a waste of $)
I would only add the stress coat stuff if you plan to change like 75%+ of your water, or if you're adding new fish to the tank. The stress coat is only there to make new fishes acclimation to new tanks a little less stressful. It's not necessary when doing small water changes...it'd pretty much just be a waste of $$$ in my opinion.
As for the cucumber...take a small slice, without any of the skin, and microwave it for like 30 seconds. Let it cool and just drop it in the tank and watch. It might take a while for your goldy to figure out its food, and it might not even like it. After a few hours remove the left-overs. If he doesn't eat it, no reason to give it to him again.
I've heard some people dropping cucumber in w/o microwaving it, but I always do...heard it was a good thing to do a while back - better safe than sorry
As for lettuce, romaine lettuce is much healthier and better than iceberg...just so you know.
As for peas, you're right on the money.
Pay close attention to how your fish is acting (it's be a good idea to get a water test kit). If ammonia and such gets too high, do a water change so he doesn't get sick and die. If he starts acting sluggish or sick, try a water change and see if he gets better. The cycling process is VERY hard on fish.