[Archonsod]

You can increase their magic levels withing the game too. Either by crafting items which grant a bonus to one or more paths, or by empowering the commander/pretender (spending gems to increase their level). The maximum level available at creation is 10, though you can increase beyond that by using items (Since mages gain less fatigue for casting spells under their level, this can sometimes make sense. It's also handy for spells which have effects or number of effects depending on the caster's level in a certain path).
Research is independent of magic levels. You can research a field all the way up to level 10 regardless of your mage or pretender's levels.
In short, the only real permanent effect of the initial path choices of a pretender are the blessing effects. These will stay set at whatever they are at creation, regardless of whether you later increase (or decrease) the pretender's paths.