I need to setup Squid Proxy and OpenVPN up on the same server, using the same Radius authentication source for both.
Both will be accessable from different ports, but my question is. Is this possible, or are there any overlaps that may stop it working?
There is no inherent conflict between the two services.
The only possible trouble you could have is as a result of any modifications to the routing tables on the interfaces that OpenVPN interacts with. However, that's not precisely an issue with OpenVPN / Squid, that's simply a matter of being mindful of routing tables.
Go forth and build your cephalopodic VPN server.