If you insist on only getting one damage dealing weapon then Iron Slug is better than Callahan by all means. That said, I don't think this loadout is a very good idea. You'll need more variety.

Yes, this loadout would work, assuming that you tailor your magnus damage type to the enemies at hand (or just use normal). But it's not great.

Freeze is very attractive to newish players, because it helps them control a room. But in my experience expert players don't use much freeze. Why? First, freeze hinders some important tactics such as knockback, shield-bumping, kiting, etc. Second, the time that you spend freezing could be used for doing damage instead. Third, as other posters have noted, your teammates often break the freeze anyway.

In general I would recommend a vortex bomb instead. Even expert players use those, for gathering up monsters to inflict massive charge damage. But there is a problem, that you may not have time to charge a gun into your own vortex.