Ok so here's the (simple) solution ...

First of all, had to activate remote printing feature if it wasn't.
This is done by running "rlp" on SCO machine.

Second, had to run "rlpconf" to add the new printer. Say it's remote,
give the host it's attached to, and not to use "extended remote
printer protocol" as my host doesn't seem to support it...

And finally restart the printing service.

And it's all ...

For those who could have the same problem, /etc/printcap is used only
to define _remote_ printers. /var/spool/lpd also.
For local ones, take a look in /var/spool/lp/



Finally, here's the config it works on :

Print server :
Linux 2.4.x kernel - Debian
Cups print server, with package "cupsys-bsd" to understand
bsd/lpd protocol. Listening on port printer (TCP/515)
Client :
Sco Open Server 5.0.5 rel 3.2

Thanks to those who helped !