So, I was thinking about the Eldritch guardian combined with Swarm Monger.
Due to both granting Familiars, they stack for everything, even the swarm damage since that is based on HD.
Now, the bigger question is feats.
As vermin familiars lose mindless and thus have feats (sadly 1), but Eldritch Guardian grants all your combat feats:
Armor feats, Caster's Champion, Piranha Strike?, etc all work great. If you have a poison, ability focus is a feat that the vermin could use.
Now, my question is Piranha strike does this apply to swarm damage?
Or does their need to be a attack roll?
The feat only disallows touch attacks and none damage attacks: swarms are neither.
I.e. can swarms get free bonus damage while ignoring the usual penalty to attack rolls because they don't make them? RAI obviously not.
RAW, I see no reason to think the swarm's attack counts as a light weapon. Light weapons need to make attack rolls, after all.
P.S. Ability Focus is not a combat feat, hence will not get shared.
A. I meant the vermins starting feat should be Ability focus as it is no longer mindless and gets feats now.
B. The swarm gets free damage with Caster's Champion as long as an arcane caster is within 30 ft.
c. Swarms attacks are just multiple bites which are natural weapons that are light. So I'd assume it would count.
But that is another FAQ question I guess.