Since you need 5 toons for a squad, ideally you're farming toons from more than one place:
-Cantina nodes
-Cantina store
-Squad Arena store
-Galactic War store
-Possibly LS/DS nodes, unless you need gear more
You can star them simultaneously without conflict in most cases. There is some overlap between gear, but usually getting to G6 is pretty easy. Same with abilities; getting them to 4 or 5 should be fast. After that, I tend to take a one-by-one approach: get a toon up to G7, ability 6, then move on to the next one until all 5 are leveled, then start over with G8 and ability 7. Of course, some abilities can be ignored; if you're never going to use a particular toon as a leader, don't waste those precious purple mats leveling up their leader ability. Mods should be assigned based on what benefits your characters; look up a mod guide to see which mods go where. If you're still pretty new, all you'll really have are health mods; focus on the primary/secondary properties and place accordingly.