How would folks suggest I "build" Jusis/Alisa/Machias? They seem like they'd be decent secondary mages beside Elliot/Emma. Gaius/Fie looks like they'd beneficiate from being Evade DPS and well Laura/Rean it's obviously pure STR.
You want everyone to have action quartz since speed is the über-stat in the Trails battle system.
Machias isn't really used as a secondary mage that often. His most common build is as a debuffing machine. People like to stick lots and lots of status ailment and stat down quartz on him (he has three lines so he has lots of room for them) Since his basic attacks hit an AOE, he has a significant chance of SOMETHING landing if he hits a group of critters and those debuffs/ailments also apply to his Petrification Shell craft which has a basic 50% chance to take an enemy out of the fight then and there. Later on, you also get access to Master Quartz that inflicts a random ailment or stat down on hit (on top of everything else) and it works wonders on Machias. (they're called Juggler and Cypher) When he gets his final craft, people who insist on breaking the game in half like to equip him with a high-level Moebius Master quartz (raises effectiveness of items) and Chrono Burst art. (take a double turn right on the spot) This allows you to squeeze infinite turns out of your team as long as you have decent magic recovery items and don't care about your personal honor stat. :P
Gaius and Fie are indeed dodge tanks. You want them to have evasion quartz, upgraded (rare) Shining quartz and attack quartz. If you decide to pick Neithardt's MQ instead of Sara's in the final chapter when given the option (you can only get one per playthrough) his Chevalier MQ is fantastic for Gaius.
Laura's thing is pure Str as you said, but Rean's thing is AT Delay rather than pure offensive power, so most players stack speed and impede quartz on him to boost the effectiveness of his Arc Slash.
Jusis and Alisa are indeed decent secondary mages, so you can give them whatever elements your primary mage doesn't have and have them chip in whenever you meet monsters you primary mage lacks the tools for. As long as whatever you face doesn't have fire immunity, the Criminal MQ is a good investment on secondary casters. That said, Jusis and Alisa also have fantastic support crafts. (well, Jusis has one and Alisa gets two) Those support crafts are their main thing, always and forever.
I personally always build Alisa as a support unit with one or two offensive spells in case she has nothing else to do. Her starting MQ is ideally suited for her and most people keep it on her all game long because it revives her if she gets knocked out which is handy because her support crafts only target her allies and not herself. (which is logical since they're CP restoration crafts) The Angel MQ allows her to keep using her support crafts even if she dies once. It also has full-screen attack arts later on and an ATS buff to boost her magic damage. I like to put a single target healing art and the Impassion art on her so she can keep her own CP up and heal herself if necessary.
As extra attack art for secondary casters, one of my favourites is Dark Matter, which is the space element (not many enemies resist it), is grid-targetted, has a good AEO and sucks smaller enemies towards its center so other characters can engage in more AOE shenanigans.
Unlike Alisa, Jusis is good (but not great) in both physical and magical attacks if you build him towards that. If you want to make him hurl attack magic, use whatever elements your primary mage lacks and make use of his Noble Command (raises attack, speed and magic damage) and Rune Blade (does damage and cuts enemies' magic defense in half) to maximize damage.
