I feel it's generally best to have 1 of each type on a team from early to mid-game, the extra damage from hitting a weak attribute is pretty significant when fighting enemies that are 10-20 levels higher than your girls. Once your girls are strong though it doesn't matter nearly as much.

Right now I have 4 different squad pages. 1st page is the helper squad page and TP page (for the TP quest and ally rankings), just sort by power and move your strongest/most useful girls in the first squad as that's the helper squad. 2nd page is general farming and clearing, where I try to spread out power equally. 3rd page is for when I need to make a specialized squad to 3* a map (fast, slow, lots of 1 type of damage), and 4th page is a single low level 2* for fighting seed bearers and letting me use multiple raid points on a boss for the raid boss quest without nuking it.

Any useful synergies are pretty obvious (shine crystal drop+bonus solar blast damage, skill proc increase+increased attack with every skill proc, etc.), use them if you can but I honestly wouldn't bother synergizing the generic stat increasing passives unless you have nothing better to do.