Setting the default gateway on the clients is necessary when you are inside the LAN... to make sure packets go via the ppp tunnel rather than eth card...
Connecting from inside the LAN is a good way of testing server configs.
From outside the LAN, yes, it would pass everything through the tunnel.
Not a good choice when you only want the LAN traffic on the tunnel
Keep the ip numbers inside the LAN allocation for external clients, this way the proxy-arp and bcrelay programmes can work properly.
Both these are used to pass packets into the tunnel that normally wouldn't be passed into a ppp link, eg ARP & broadcast packets.
You can use different ip subnets, it will work, but you need static routes in the clients if you don't use the default gateway into the ppp tunnel...
bcrelay is a compile option when you build pptpd, --with-bcrelay.
It may not be present in prebuilt binaries...
Check ps ax
and see if it running on the correct interface.
Also, do which bcrelay
to find it if it isn't running. (/usr/local/sbin/)
The source package of pptpd has a file README.bcrelay explaining it's operation etc