let me try to answer your questions.
First of all let me tell you that in larger networks it is always recommendable to use Switches instead of Hubs, especially if you have such a number of nodes and a high traffic load, like HMI Systems have.
Well, OK, seen from the connection point of view I would ask firstly what Protocol is used on both Networks. In case of ISO Transport I would say no problem. You will not have to change anything. If you use TCP/IP it would require either to adjust the network settings or to use a Router to connect the two Subnets. The Router would work on one Network but with the two logical Subnets.
With S7-Connections, as you work with as far as I see from you explanations, it depends if you address with MAC Addresses or with IP Addresses.
As you see it is not really easy to say.
I hope I am able to help a bit.