Hi Beechey,
I've sent you a PM with some links. There's quite a few species of whiptails and twig cats out there. To me Royal Whiptails are a more dramatic looking fish, whilst mine are laid back characters they still have personality. They know there feeding time and are always in position on the sandbed at the front of the aquarium for the wafers and courgette. They also tell me off (or it seems like it) if I 'eyeball' them too much. Size wise, in theory the Royals can get up to 20 cm SL (with another 10 cm or so of tail). I havn't kept the common whiptail so I'm afraid I can't comment on that.
I obtained mine as virtual babes (4 cm SL - now about 12-13 cm SL) 2 and a bit years ago from Maidenhead Aquatics Aylesbury. They had an adult pair which regularly bred for them. I've not been there for a while so I'm not sure if the pair are still there.
All the best,
Triskele