My question is if its an isolated vlan (a vlan that is not routed), can I use any IP's inside the vlan.
Example, if the vlan is Vlan23 and I have put machines in there with IP addres (10.0.0.1/24, 172.16.31.6/24, 192.168.10.1/24),
there should be able to talk to each right ? since its a pure L2 environment.
I believe it should work but I do not have a lab to test this.
An IP from a subnet A will not talk to an IP on subnet B, because to allow communication between different IP subnets you require a router (or routing device).
If on the same Layer 2 segment you have devices with different IPs (from different IP subnets), they won't talk to each other.
If you have a Layer two segment and you have a device with the IP 10.1.1.1/24, it will only be able to communicate with other devices on the 10.1.1.0/24
If you have a directly connected device on the same Layer 2 segment with an IP from 192.168.1.1/24, they won't communicate, because the first IP packet between the devices need to be routed (there's no routing concept in a single Layer 2 segment).