Hazards of increasing network size by changing the netmask

Maybe not the best design:
Are you sure that is the route you want to go? It should work fine, but then you are using the entire 192.168.x.x network for one broadcast domain. That is going to give you 65534 hosts. If your company grows, you are going to have re-subnet the entire network again or start using 10.x.x.x or 172.16-172.31.x.x address.

A More Standard Way to Approach it:
I think a more normal approach be to use a subnet per location, or office floor, etc.

1. Start with how many machines might end up at each location and then maybe bump up to the subnet above that.

For Example:
So if you think you might have 254 devices on the network per area, bump it up to 255.255.252.0, the next subnet up. This will give you a host range of 192.168.0.1-192.168.3.254. Then when it becomes time to expand, the next range using the same subnet mask would be 192.168.4.1 to 192.168.7.254.

A Simple Way:
A simple way if your office is small, would be just to set the DHCP range to something like 192.168.0.1-150, and then use the rest of the IPs in that address space for static assignment. Your DHCP server should have the option to specify a host range to hand out that doesn't line up with network mask. And if it only works by subnet masks, you could always cut the subnet in half with a mask of 255.255.255.128.

A Good Subnetting Read:
I recommend reading Evan's answer to How does Subnetting Work, and How do you do it?.

You need to add your subnets as trusted subnets on your firewall

in juniper - I think it is done via virtual router interfaces ; so you may have to create rules of any - any from either of the subnets to each other.

So on 64 network interface you will add 65.x.x.x as a trusted subnet allowing any-any traffic in both directions and the same on 65.x.x.x interface.

Check your firewall KB or manual and it will tell you which mode it should be in - i think it is the "route mode"