I would give some thought to putting the puffer in a tank of at least 10g/38 liter with maybe the clown pleco cat,
by the way, though they are fairly slow growers striped talking cats can reach 8" long,
in your 12g/45 liter tank I would retain your remaining siamese fighter (betta), 5-6 neons, 5-6 glowlight or other small tetras or guppies, 3-4 zebra dianos, your cory cats, and your tiger barbs which will pretty much max it out for long term.
In your projected 25g/95 liter tank I would keep your remaining fish plus maybe 3 black skirt tetras and 3-5 silver hatchet fish and that would be about it. I would also add a couple bunches of anacharis/elodea plants to each tank.
There are many types of Malawi cichlid, some are a bit aggressive and some are not, and they range widely in adult size. Could you give us a picture or a very detailed description?
Bettas are called siamese fighting fish because they are very aggresive towards each other, so much so that staging fish fights is a popular betting sport in some places. They should not be kept with other bettas.