Recommendation For Hospital Tank

Unsure if you mean you're planning to get a hospital tank for the purpose of treating fish on occasion when required or if you're planning to get a hospital-sized tank with the express purpose of keeping shrimp in it instead of in the main tank.

If it's the former, than plastic can be a good idea, as they can be more easily stowed away (eg at a height) when not in use and possibly with other fishkeeping items in them. However, I would definitely not recommend The Range's Rosewood plastic tanks, even in the unlikely event that you managed to find an unbroken one - the lids don't accommodate the cable wires. I managed to find a replacement plastic tank from MA which is good but it was overpriced for what it was due to marketing/featuring a children's film - it has a very compact but effective filter, though; the only downside is that it has a curved front on it which seems to agitate the fish when in it for the first 24+ hours or so, most likely due to not being used to this.

What would be a recommended size for a quarantine / hospital tank?

For me, the largest fish would be a single BN Plec, or a few full size dwarf rainbow fish, or maybe even a couple kribs (as a hospital tank).

As a quarantine tank, perhaps a dozen juvenile harlequin rasboras or half a dozen kuhli loaches.

Is there a formula I could use, or is it best to also follow the min tank requirements in the fish profile?

I know it's not 30 x 60cm, but would this be a suitable hospital tank, for my jeuvenile dwarf rainbow?
http://www.petsathome.com/shop/en/pets/fish/tropical-fish-tanks/love-fish-vantage-48-litre-aquarium?orderBy=3

This is another possibility. And potentially half the price for a larger tank.
https://www.fishkeeper.co.uk/product/superfish-aqua-cube-sets

I'm just negotiating with Mr Helen to put the 'spare microwave' in the garage, so I can replace it with a hospital tank. If I succed, not only has it given me an excuse for actually getting rid of a microwave that gets used once a year, but I'll have a more permanent home for the qt / hospital tank. 

If I fail, I'm not sure where I'll put a tank!

The first one looks fine to me - provided you remember to take the carbon pad out of the filter. And I'd also remove the algae pad and just put plain media in there. You don't want anything that could possibly remove a medication.

The second one looks to be open topped; the fish will be stressed and stressed fish jump. And it is also better for a hospital tank to be kept without a light, and I would be concerned that an open topped tank would be brighter inside than one with a solid lid. If you did go for the second, I would cover it with something.

If it has a glass top, that's better. it wasn't clear from the description. You could use an old towel on top to make the tank dimmer.

Some medications work better if the tank light is left off.

It's not something I particularly want little helpers for!

Especially as it stains. Being distracted at just the wrong moment could result in green carpets and clothes 

Which would be the better substrate to use? Sand, fine gravel or normal sized pea gravel? I have all three left over, so can pick whichever would be better. Or nothing if that is better for a hospital tank. Though I feel that my rainbow would probably prefer a darker substrate.

I actually have loads of spare substrate left over from when i refurbished my main tank. So I wasn't planning to take it from the main tank, just use some of my ample stock! Therefore I wouldn't need to worry about returning it to the main tank.

But I don't have either shrimp or snails other than pest snails (though i don't mind them because I'm convinced the kuhlis eat them)

The filter has loose carbon inside a foam sponge. Do I just replace the carbon with ceramic media or take out the sponge as well?