Neither are showing clear signs of being male at the moment, but they do look quite small so too young to tell

You're really looking at a bigger tank than that for an oscar - 5ft x 2ft minimum, so I would stick to something a little smaller. Green terrors are extremely aggressive and shouldn't be mixed with other fish (except as a mated pair) in less than about 650 litres. Silver dollars need at least 6ft in length.

Bag it in water from the tank it's in and then acclimatise by floating the bag in the main tank and adding a little water every 10 minutes or so. After about an hour net the fish out of the bag and into the tank.

Yes, it's normal, but the smaller one may suffer stress if the big one hassles it too much.

It's a shame there was a problem with the first tank, but good to hear ND still has excellent customer service. Enjoy your new tank (when you can)!

https://www.fishkeeper.co.uk/stories/coronavirus-updates

During the pandemic remember it may be difficult to obtain everything you need for your fish as shops close and the public are asked to only venture out for essentials. With that in mind here's a list of things you may want to do to ensure your fish remain happy and healthy.

Make sure you have enough water conditioner for those regular water changes and enough for extra emergency changes if necessary. Consider buying Seachem Prime which doubles up as an effective neutraliser for ammonia and nitrite in the event of spikes in either.

For those using RO, ensure you have enough for a few water changes at hand, along with plenty of your usual remineraliser.

Check your fish food supplies and replenish if necessary so you don't unexpectedly run out.

Have basic medications at hand as if the shops close, the extra day or two saved by not having to order online may make all the difference. A good anti-bacterial, anti-fungal, and white spot treatment should suffice. Esha 2000 and Exit or your medicines of choice.will cover these.

Consider keeping a backup heater handy in case of failure - cold fish will also be immuno-suppressed so having a spare on hand rather than having to wait for delivery can be very useful.

Finally, don't forget that during this time, your fish can be a great source of relaxation and help you to de-stress!

These look too young to sex reliably at the moment, but if I had to guess I'd say female at the left, male on the right. More pics showing their dorsal fins would help.

It's very hard to say without more information.

What size tank was he in?
On his own or with other fish (if so what)?
Water hard or soft?
Temperature?
Ammonia, nitrite and nitrate readings from the time he was showing symptoms?

Swim bladder issue possibly caused by either constipation or an internal bacterial infection. You can relieve constipation by feeding thawed frozen peas, squeezed out of their skin, or daphnia. If that doesn't help treat with Esha 2000.