Unlocked phone, old sim, use witb att data plan

No, you can’t use a phone on a data only plan. 

If I have an unlocked phone and sim,

Deactivated SIM cards cannot be reactivated on AT&T. 

Not all unlocked phones will work as phones on at&t since 3g shut off. 

would i be able to give you the imei and iccid

Noooo... this is a customer populated Forum you should never publish that sort of information here

and using that phone and sim can I purchase a data only plan thru att,

No.  AT&T requires that devices be used on a plan that is compatible with that device. So phones on a phone plan. Tablets and hotspots on a tablet or data only plan.  

to use that phone and sim as a mobile hotspot?

Nope. 

Yes the phone is compatible with AT&T at all, it would have to be used on a phone plan. AT&T's best phone plan which offers hotspot capability would be the Premium plan which for one line is 85 a month after autopay and paperless billing discount. That plan offers Unlimited on the phone itself and 50 GB may be used for Hotspot.

AT&T currently offers nothing on postpaid consumer plans for a dedicated hotspot and unlimited data. They don't even offer anything with a lot of data on postpaid plans. 

AT&T's prepaid data only plans which are available for dedicated hotspots or tablets offer up to 100 gigs for $90, for 30 days.