[JustOneMore20]

The Apistos (Co-ckatoo Dwarf Cichlids) would be fine with the Gouramis, as would the Rams. Cichlids can be territorial, especially when breeding, but there should be enough space for both the Apistos and the Rams to have a territory. Rams don't school...they can be kept in pairs and may even breed.

CAEs can be aggressive as they get older so I suggest returning it and getting 6 Otos once the tank has been setup for a few months.

I don't recommend keeping Discus in a 55g. They get big and a 75g would be better for the width. They are also skittish and need good water quality. They can't be kept with just any other fish.

Either substrate is fine. If you want to use some of the 30g gravel for the cycle, but would rather use sand in the new tank, you could put the gravel in a nylon stocking or mesh bag and just sit it in the tank for awhile until you are getting "cycled tank readings" from your test kit. Then you could just take it out.

If your pleco is a Common Pleco, it will eventually outgrow the 55g (they can get about 2 feet long) so I'd go ahead and exchange it as well for Otos if you can. If its another type, then just make sure it stays less than about 8 inches long full grown.