> ibiscus---------- margo ---------- cactus
> 
> ibiscus and margo are linked via sl0 (slip). margo and cactus are linked via
> ep0 (ethernet). cactus is my gateway to Internet.

 Change slip to ppp, makes thinks easier, ie. default routing and proxy-arp.

> From ibiscus, I can ping margo and not cactus.

 Missing route on cactus (route add cactus margo), alternatively 
proxy-arp on margo.

> From margo, I can ping ibiscus and cactus. Kernel compiled with GATEWAY.
> From cactus, I can ping margo and not ibiscus.

 Same as above. 

> How to make things so I can ping all machines from anywhere, and acces to
> Internet from any machines ?

 You can't unless you providers knows about margo or ibiscus or you install 
IP-NT.

-- 
Markus Illenseer