Can i collaborate with 1.91 && 1.95 without any version conflicts.

Afaik, yes, according to the freeswan users list. I've got a 1.94 and 1.91 cooperating, even with 1.94 having some known bugs... As long as you don't try to use features only available in post-1.91 frees/wan it should work. I'd not use the "opportunism" stuff.

And - by the way - the security-patches of grsecurity.net - do they love freeswan like me ?

Don't know. Get a VPN without the grsecurity stuff running, and then add it. Tell us if it works, please. Robert