Most of us feel that summons sort of get a bit weak after mid-game and are useful only as distractions. What I feel is that the higher your wizard level, the stronger variant of a monster you can summon (if it has one) or just improve the statistics the existing summon has. For instance, the Animate mustache spell of the Golemancy tree can, after wizard level 10, summon up a slightly stronger version of a Grandpa Mustache instead of a plain old mustache spirit. The wyrmling/zombys/etc can benefit from boosts to stats and damage. Stronger versions of the blobbies can be summoned using fungal arts, like Thermoblobbies at level 5, for instance. Just throwing this out to see how relevant it can become.
Summons in each archetype should scale with levels in that archetype. Hunting diggles are great for a warrior to have at their side a temporary choke point that fights for you pretty much. Only a little though since the distraction factor alone has considerable value.
Look at the Summoner skill mod. This is right in line for that. I like the idea. I just have no idea how to make it work correctly without being both over and underpowered.
Grant your summoned monsters a buff - and have the benefits of that buff scale with the user's magic power.
Is it? I don't know, it depends if you want summons to ramp up constantly or only when you're gaining more caster levels.
Magic Power is better. Don't question it! Seriously though. Just because you don't have a ton of Wizard skills doesn't mean your summons shouldn't improve as you level up. In fact golemancy is perhaps most useful on characters that otherwise don't have many wizard abilities.
You can always try to get a few more points of magic power with any character, but getting more levels of wizard skills might sometimes not be possible.