Can we place infrastructure and GC on same DC?

Yes, you can place IM role & GC if one domain in a forest. Otherwise, you need to move IM role to another server if in case multi-domain environment it needs to update the cross-domain object references on each DC.

yes you can if you have one of the following scenarios

1- All Domain Controllers are Global Catalog

2- you have only One Domain in the Forest

No, Because both them doing same role.So anyone one of the role  place on DC.

Regards,

Vijayaraja.A