Can I allow Mac using Internet Sharing via Thunderbolt to talk to other devices on network?
Enabling Inter Sharing on your iMac makes it a NAT router. Its main purpose is to connect the downstream networks (192.168.2.0/24) to other networks (like your main router connects your internal network(s) to the internet). 

Your MacBook will connect to either other devices in the upstream network (192.168.1.0/24) or the Internet.

If you want to share a service on your MacBook (e.g. an Apache server) to the upstream network (192.168.1.0/24), you would have to forward the respective port on the iMac and add routes to the networks 192.168.2.0/24 on all other devices in the network 192.168.1.0/24.

Assigning an IP address of the 192.168.1.0/24 to the MacBook isn't possible.