You can basically have every class (later on turns into a job) all on the same character and easily changed by switching out your weapon.
All jobs used cross class abilities from other classes so you will eventually want to level multiples anyhow.
Like for example Archer has Quelling Strikes which reduces agro and Raging Strikes which increases damage. I use these abilities on my Smn and Black Mage.
Beauty about you starting off Arcanist is you can level the 2 jobs at the same time since Summoner and Scholar is sort of a "shared" class, makes leveling up so much quicker and easier.
For bonus stat allocations (in character menu) you will want all in dexterity for Rogue/Ninja, Arcanist/Summoner would be intelligence.

Welcome to the game