Here are some answers for you:
1/ how soon after the cycling has finished do I need to get fish in?
Once the cycle has finished you need to keep your newly raised bacteria in tip-top condition. This means they need to be fed with ammonia. Either from the bottle of ammonia you have used during your cycle, or "au naturel" from the introduction of fish. So, in theory, you don't ever need to introduce fish, just keep bottle-feeding the bugglies. Quite why you would wish to do something so perverse is beyond me but...
2/ will it be a problem if I only put a small amount of fish in to start with?
No... But... The bugglies in your filter will adjust their population according to the naturally occurring ammonia load; Few fish - low levels of ammonia - few bacteria. When you come to add further fish, you should only be adding around 25-30% extra fish at a time. The bacteria need time to build up their numbers to handle the higher bacteria level. So 25-30% of 3 fish is 1 fish; 25-30% of 30 fish is 10 fish. You add more fish more quickly. Your choice. Go at your own pace.
3/ how many fish can I stock in my tank? It measures 50cmx33cm and is 40cm high. If I'm right I can only put quite small fish in this length of tank but I really don't have a clue how many.
You are right, you can only put smaller fish in this size of tank. How many? Good question. ... ... No it's not, how many fish depends upon a lot of factors; What sort of fish is the main one. Male Betta splendens the answer is 1! Microrasbora sp. 'Galaxy' the answer may be as many as 10! Try and investigate your options using the Community Creator elsewhere on this site.
Thanks for reading !
No, thank you for asking...