I have a 55-gal running for just over a week now. After letting the
water circulate for a couple of days, I put in 4 or 5 gouramis.
Everything looked fine, so I put in a bunch more over the weekend.
The only advice I received from the store was to change 20% of the
water per week for the first 2 months.

Now that I've spent some time reading the postings and a few dedicated
sites, I know learned about the nitrogen issue, growing new bacteria,
etc. I really hope I can manage this cycle and not lose the fish in
the meantime. There are around 16 small gourami variants, plus a
small blue lobster.

Should I mainly focus on water changing or try one of the accelerant
products I've seen mentioned?

The tank has 2 powerflo pumps feeding an under-gravel tray, plus an
Aquacleer 300 carbon filter running continuously. The plants are
fake, and the gravel was purchased new. The tank came with large
corals that had been used by the previous owner, though they had been
sitting dry in the garage for several years.

Thank you for any advice.