Interacting with other clients
 

I am pretty new to file sharing and tried Limewire a few weeks before trying out Phex now for 2 or 3 weeks.

After having sorted out my firewall I have been able to connect as an ultrapeer, but I notice that only certain clients will accept me as an ultrapeer. Among others Limewire clients after 4.8.1 seem to reject me. I was wondering if anyone has any explanation to why this happens? Does it have anything to do with Phex, or is it my network setup?

Klark

While Phex accepts connections from all clients without restrictions, Limewire generally preferes other Limewires over other clients. In some versions more in others less..

Bearshare even only accepts UP2UP connections from other clients.

When acting as a UP you usually should have no problems finding other UP. Maybe a small warmup time is necessary, when Phex wasn't running for a longer time.
Phex actively searchs for UP and advertises itself to accept Leafs.
Finding Leafs (or better Leafs finding you) is a bit more difficult because of the vendor preferencing reason. Leafs from Limewire always have the tendency to prefere other Limewires and might only accept other hosts if they have problems finding Limewires. Bearshare Leafs will never accept any other client.

Is there any way to spoof the LW clients into thinking you're a LW too?

In theory yes, for Phex no.

Phex will always do its best to be a good Gnutella citizen.