I don't like the idea that the ruin reward may influence the ruin guardians: it is a very fine grained and very special rule.
Only expert players can take advantage of such odd speciality rules (and they will take advantage), but everyone else, especially beginners are kept out. Keeping it simple is usually a good idea, also from the viewpoint of coding.
Increasing the number of allies is one possibility, another one is changing the guardians for all ruins. I believe the rounding error problem with skeletons is not yet fixed, is it?
How about mixing the undead types with a formula like this:
L1: 1-2 Ghouls + 0-2 Skeletons
L2: 1-2 Mummies + 0-3 Ghouls + 1-3 Skeletons
L3: 1-2 Vampires + 0-3 Mummies + 1-3 Ghouls
(or in D&D terms, an L3 contains 1D2 Vampires and (1D4)-1 Mummies and 1D3 Ghouls.)
So all L1 ruins contain at least one Ghoul, all L2 at least one Mummy, all L3 at least one Vampire.
This also fixes the issue with rounding (there is always a Ghoul with its Protection-from-rounding-errors ability). Formula like these are also easier to explain, instead of the complicated-looking probability tables that we have now (which are fine for me, but which may put of beginners at first glance).
It might also be quite exciting to watch the ruin battle when your hero enters and first easily defeats 2 Skeletons, then bravely bests 3 Ghouls (but loosing a hitpoint) and eventually dies against the single Mummy remaining.
With the current system, the tension is immediately relieved for me if I see the undead type.