Sounds like you have an awesome LFS - stick with them, they know what they're talking about.
Biorb 60 will give much more space for your fishies - have you a biorb 30 now? Make sure you carry over sponges and media from the old one to the new so you don't end up with new tank syndrome again. 60L is still a little tight for two goldies (ideally they should have 10 gallons each, approx 40L) so be prepared for another upgrade as they grow, but should be fine in the interim.
Stress coat and zyme is fine (they're API brand) - I used to use those. I switched to Seachem because I discovered that Nitrates were high out of the tap (and I switched everything to Seachem - I have a thing about mixing different branded water treatments... I've heard that occasionally they can do unpredictable things, so best to stick to ALL API or ALL Seachem to be on the safe side).
30% daily water change should be fine as long as you don't go much above that, the fishies have time to acclimatise - although thinking about it that's also diluting your medication for your fish. How much above is your nitrate levels now? If it's masses above I'd stick with the 30% as getting water quality back should be priority. If it's only slightly above maybe drop to a 15% water change.
Aquarium salt can be good stuff but I know what you mean about the dosage. Don't go over what's on the packet for the swim bladder stuff. I would highly recommend Melafix to supplement as it's a general pick me up for fish and I've had more success with that than any specific treatment, to be honest.
As I say with the other food, it's a bit of an art rather than a science. As I said before, I tend to try and give frozen foods twice a week instead of the flake/pellets and fresh once a week - but I know others have different routines. It depends a lot on how well your fish enjoy the alternative foods. My tropical fish go nuts for the frozen foods, but aren't that keen on fruit/veg (but the snails adore the cucumber!) Play it by ear, but I'd say at least once a week have an alternative for a bit of variety.
The last bit I hate to add, and I really want to stress that I don't think you'll need it as he's still eating well, but I think every fish keeper should know it for reference. If he gets very very sick and sluggish and it looks like he's not long for this world, please don't flush him.
Most times it's taken out of your hands, and if the fish dies it's really important to get the body out asap as the other fish will probably try to eat it (and any infection will then pass to the other fish).
However, if you feel the fish is in a lot of distress and there's no hope of recovery, there are lots of suggested ways to euthanize a fish - the 2 kindest and quickest are:
- remove from tank and chop head off quickly with a knife. (No, I know... I have NEVER EVER managed to do this).
- remove from tank and place in a small container (old marg container works well - something you'll never use again) with some tank water. You can add a few drops of clove oil (can buy from pharmacy, also good for tooth ache apparently) to dope the fish and then top the container up with cheap (high proof) vodka (you're looking at having 50:50 tank water to vodka). Replace lid, don't look, have a cry, come back later and dispose of body and container. It's swift and painless for the fish.
Hopefully you won't have to do that, I've only had to do it twice (I had suspected case of fish TB and it would have been a long slow death wiping out the rest of the tank in the process), but I think every fish keeper should know what to do and have the relevant supplies in just in case.
Sorry to end on a morbid note. As I say, I'm sure you won't need it - fishie is feeding which is a great sign, he's just got a bit of a bloated and uncomfortable tummy!
Keep me posted!