i'd put fossil+venti on a separate machine from a general
cpu server, much as with the old file server.
that way you can give it lots of memory all to itself, for cache, and you are
much less likely to suffer unpleasantness (because a random program
exhausts memory for instance or trips over something in the system).

i'd have thought the auth could happily coexist, though:
they are small, fairly easily bounded as to the resource requirements,
and are unlikely to cause much trouble.