Question - is the orangey yellow fish near the bottom of the heater one of the gouramis? If so it is a honey goorami, not a dwarf gourami.
You do have a small problem with the fish you have now - bettas and gouramis do not make good tank mates. They are closely related and sooner or later there is a very good chance of fighting between them. Bettas are well known for appearing to be happy with their tank mates, then they suddenly snap and go on the rampage. If you want to keep them together, have a back up plan. Either another tank the betta can be moved to, or as a temporary measure one of those nets for keeping fry in. You will need to make a cover of some sort for a fry net as bettas can jump, especially if there is something on the other side he sees as an enemy.
So to your question about more fish. The first thing I would get would be another glowlight tetra, and assuming the small suckerfish are otocinclus, another two of them as well as they are shoaling fish. But if you have something like a bristlenose they get too big for your tank.
Those extra fish will put you are fully stocked I'm afraid.
The real problem is the betta. They are not good community fish and it is tricky finding fish suitable to share a tank with them. Both neon and glowlight tetras can be iffy as they are likely to nip the betta's fins, and as I've already commented, gouramis are not a good choice either.
However . . . .
Do you still have the Edge? If you do, that would be a good tank for the betta provided either he can find the small square of air to breathe or you keep the water level lower so there is an air gap under the top glass (yes I do know it will spoil the effect
)
That would free up space in the 64 litre and make the choice of other fish wider than with the betta in there.