equipment needed for 55 gal tank

Hiya
Sounds like you've got almost everything. I highly recommend Rena FilStars - they're great filters, and not overly expensive.

If you're not going to keep live plants, the amount of gravel is really up to you. A layer of gravel 1" at the front sloping to 2 - 2.5" at the back should be enough, and easy to vacuum clean.

As for other things you may want to get, here's what I can think of off the top of my head:
- A net (you never know when you'll need to catch a fish)
- A bucket reserved for aquarium use
- A sponge or algae magnet for cleaning the glass
- A thermometer (I prefer liquid crystal stick-on types)
- A variety of foods (it's good to get this at the same time as the fish, the store can tell you which foods are best for the fish you're purchasing)

Before you get your fish, it's a good idea to look up species profiles. That way you'll know you're getting fish that are compatible with each other and the conditions you'll be providing, and they won't be species that will outgrow the tank.
I don't recommend clown loaches for a 55g - they can grow to 12" long and need to be kept in groups. There are lots of similarly shaped loach species that stay small enough for a 55g, it's probably better to choose one of them.

Have fun

Unless you're going to age your water every PWC you'll also need a dechlorinator to remove chlorine/chloramine from your water. Brand doesnt really matter.