In general will that "ruin" the intended build?
Not really. Oath only changes a few things like what you can use charges on, what spells you get and what aura you get.
Spells are generally a non-factor for Paladin builds who typically use spell slots on Smite. Meaning it's really just aura and Oath Charge usage that differs.
Or are most build suggestions more adding it for the base proficiencies? Does Oathbreaker deviate from say just a Vengeance build that much?
In general there are a few things that builds pick Paladin for:
- Proficiencies
- Smite
- Extra CHR scaling on weapons
Oathbreaker and Vengeance both do the same things here. Oathbreaker gets its level 7 aura to apply its CHR modifier to its melee attacks (As well as nearby undead and fiends so buddy up with a Necromancer/Spore Druid for funsies) while Vengeance gets Inquisitor's Might which lets them use a bonus action to add their CHR modifier as Radiant damage to their attacks with a chance to daze on hit too.
Also, if my main character does something "questionable" does that cause my companion with the Paladin build break their oath by association with you?
Depends.
Some major actions makes them guilty by association (Things like using Ethel's wand or liberating Sazza can break companion Oaths), minor things can be overlooked.
Buuuut... You can simply walk your Paladin companion away from the group if you're about to commit some serious sinnage. What they don't know, can't break their Oath.
