i removed roundcube before posting here, otherwise i could not reach ispconfig at port 8080, so ispconfig is in a good shape now. i would skip squirrel webmail installation since i don't need it. the server is not in use yet but i created some mail users and tested send and receive with pop and imap and it works ok.
now, in the tutorial you pointed, at step 2 you mentioned you created roundcube's db in ispconfig. that means you created it in 'sites' tab at database section(as you would do for a website)? i'm at that step now.
Thank you for your quick reply!