Prepaid questions

1.)  Do they allow roaming on other networks?

Not domestically.  AT&T prepaid can roam for free in Canada and Mexico. There are a very select number of places outside of the country but you can roam for a $35 per week fee. 

2.) I am interested in the 8gb online plan.  Is this 8gb data per month?

Yes

3.) Does AT&T allow wifi calling?

With compatible phone, yes

4.) Can the phone be used for as a hotspot?

Yes, all 8 gigs if you like 

5.) Can you keep your old phone number.  I currently have a Tracfone.

Yes.  You port the number to AT&T.  Ask tracfone for account number and security pin 

There is some very limited domestic roaming in the US.  If you zoom in on the prepaid map on ATT's site I found some in NE Iowa, SW Wyoming, etc. 

I don't know whether it extends beyond just voice and text.  Data roaming has often been limited.