Looking at each sentence in turn:
It was the first meeting since the graduation.
This is OK grammatically, but it is a little impersonal. Suggested alternatives:
It was our first meeting since graduation.
It was our first meeting since we graduated.
It was the first time we met since we graduated.
2) I've seen them for the first time in 20 years / in a long time.
This is perhaps the most interesting (and problematic) of your choices since present perfect doesn't really work here. Viable alternatives:
I saw them for the first time in 20 years.
It was the first time I'd seen them in 20 years.
It's the first time I've seen them in 20 years.
I saw them after 20 years / a long time.
OK gramatically, but it sounds like you are recounting it years later. Could be something like:
I hadn't seen them for 20 years.
It's been a long time to see them.
No. Should be something along the lines of:
It's been a long time since I last saw them.
It had been a long time since I'd seen them.
... depending on what you are trying to say (and when the event you are talking about actually happened).