My friend just gave me is 20 gallon tank. Looking to get started on this new hobby! Few questions

So a friend of mine gave me his old 20 gallon tank since he upgraded to a 55 gallon tank. He gave me his filter and any old decorations I may need. I gave everything a good rinse and it looks like I'm ready to get started. Questions:

1. He's been very successful with his fish. They have grown huge and look pretty happy and healthy. He told me not to worry about cycling (because he never did), and that a quick start bottle from the store will work just fine along with letting the tank run for 1 or 2 days. He also never runs any water tests. Is he just getting lucky? Can anyone confirm?

2. Betta fish are just awesome. But I also want a couple of different fish in my tank. Would I be OK with 1 Betta and a couple of smaller fish? Can you recommend any specific ones? The shark ones look pretty cool as well.

3. Gravel first, then water, then decorations...right?

4. In regards to question #2, is it ok to have little crabs or snails in there too?

5. In your experience, is equipment and decorations cheaper at a store or on Amazon?

6. The tank came without a cover... will the store sell them usually?

I appreciate the help, tips, and responses! Thanks!