the topology is:
3750 -- 2950 -- host(20 stations)
3750 support Private-vlan
2950 not support
3750's f0/37 connected to 2950's f0/29
f0/37 configured as isolate port
and don't config the 2950.
if the 20 hosts can communicate with each other on the second layer.
If you set private Vlans on 3750 and 2950 does not support it, then hosts in the same vlan will be able to communicate with each other throught 2950 switch (layer 2 traffic).
If you want to create a fully private vlan topology then you need to configure it on both 3750 and 2950 switch (upgreat IOS to one that supports pvlan).
Hope that expalains why, if yes, please rate it.