Hi Ross!
Sorry, that you are having difficulties.
You are not required to buy anything else. You can buy whatever collections you want to have. If you only want the C Trumpet, then this is exactly what you get. But of course you cannot expect the trumpet to play flute, violin or cello sounds. [;)]
As you noticed you can activate more instruments with Sibelius. So if you want a flute, please activate whatever instruments you might have, to play flute sounds.
You will understand that I can't give you instruction how to use non VSL sounds. I don't even know what instruments you have.
About your latency problem. I heard that Sibelius has some problems in the latest version with latecny compensation. This occurs only when mixing VSL sounds with other sounds. If the VSL sounds are late, I recommend doing the following:
Select your VSL staves (in your case the trumpet) by trippleclicking them. Activate the Sibelius Properties window by Clicking on Windows/Properties. Go to the Playback section. Select Live start position and enter a negative value, so that these staves get played back a bit earlier. You might have to experiment a bit to find the best value.
Did you create a trumpet matrix with the patches you need for your score as described earlier in this thread? This is required, if you want to hear different articulations and not just one sound.
Don't get frustrated to quick. I think you are almost there.
Best wishes and merry christmas,
Andi