The usual advice is to soak wood until the worst of the brown colour has come out of it. What is the water like in the bucket after it's been soaking - how brown is it?
Wood can turn the tank water brown but the fish won't mind that. It is us humans who don't like a funny coloured tank. If you put your new wood in the tank and it does turn the water brown, water changes will slowly get rid of it, or you could put carbon in your filter. If the water goes very brown you would need to change the carbon every few days as carbon gets full quickly.
Some wood is worse than others though. I have a piece of wood that was sold as mangrove root. The woman in the shop said that kind didn't need soaking, and she was right. There's never been any brown from it.
The holes - are they big enough for your bristlenoses to get stuck in? If they are, you could try using silicone sealant (aquarium stuff, or DIY sealant that says it is safe for aquariums - if it doesn't say on the tube it is not safe). But you'd have to let the wood get completely dry first and wait 48 hours for the sealant to set before getting it wet again. Or plug it with a bit of filter wool, though that might look a bit funny.
Squashed baby assassins shouldn't cause much harm. I probably squash loads of those tiny flat snails when I move anything in my tank. If the shells get broken and any fish or other snails can get to them, they quite likely just get eaten.
Your tank is 160 litres isn't it? If I've remembered right you do have room for more fish.
If you choose barbs, get some that don't nip too much. Look at something like cherry barbs or 5 band barbs. A lot of barbs grow big and wouldn't look right with your tetras; some like coolish water, and some like tiger barbs would nip your other fish.
Red eye tetra - a bit bigger than your other tetras, and can be a bit nippy
Black phantoms - should be OK. Mainly nip fish with long fins like guppies
Flying fox - one of those should be OK, but shops often label these fish wrong. There are two or three fish that look almost the same and shops mix them up. Some of the others can be aggressive so you need to look up how to tell the difference.
Maccullochs rainbows - I'd never heard of those so I had to look them up. They sound like they should be OK - if you can find them. Have you seen any in shops? If you can't find any,
these rainbowfish would go nicely instead.
Glowlight danios - fine, so long as you mean
these and not glofish. Glofish are genetically modified and they aren't allowed in the country (though that won't stop some shops)
Golden wonder - nice as they are, I don't think I'd risk those with the fish already in the tank.