I've cycled my tank (which happened much quicker than expected, possibly the second hand filter still had the bacteria in?) and according to the water tests from the last few days I'm ready to add fish.
Is it best to introduce one species at a time to the tank or can I add a couple of different types of fish? Are there any species which are better to add first? I intend to have guppies, neon tetra, a clown pleco, platy and some cores and possibly some barbs and/or zebra loaches.
Is this the tank you set up a couple of weeks ago? It won't have cycled yet. What results have you been getting? The second hand filter won't have had any bacteria, it dies off within about 24 hrs of it being switched off.
it's best to add fish gradually to avoid an ammonia spike, so one shoal at a time (it doesn't need to be one fish at a time) and keep an eye on the levels for a few days. Usually you can leave it a couple of weeks between adding stock. Have you got quarantine facilities for when you add new fish?
All those fish get along together fine but guppies, tetras, platys, corys, zebra loaches and barbs all need to be in groups of 6+ so you wouldn't have space for them all. I'd probably go with one shoal of bottom feeders (corys or loaches) and then a couple of shoals of mid-water swimmers. Assuming your water is neutral-ish then you can pick and choose from what you've listed. Some barbs can be a bit nippy so be careful if you choose guppies, their tails can be tempting for some fish.