Thing is, a 40g tank would be already too large for the animals i have mentioned earlier and they would actually have a shorter life expectancy there. Some animals are doing better in a species tank without faster tank mates. Other animals are bad for large tanks because you never see them.Personally/my opinion
I would not add any fish to this aquriam, I thought mine was small at 40g 200litres.
I understand you have another aquriam set up so I assume you know about 'fish stress'
Maybe disregard this 2.5g as a setup for marine life and maybe build/create another?
Anything you add to this 2.5g will have a very short life expectancy.
I have seen my pompom crab five times in 2 years in my 15g tank.