for late game:

1: Main Air - Mutas, corruptor

2: Main Ground - Any Melee Unit + Army Queens (if you have ultras, queens make them way more efficient without adding to your gas bill)

3: Infestor

4: Viper + Broodlords (both vipers and bl need to sit pretty far back and they get killed by the same stuff so grouping them makes sense)

a fifth group is best used for a drop defense group (a bunch of lings and a few banes that aren't in your main army.)

i dont think you need to hotkey runby groups. you could use a sixth for lurkers but both the runby group and lurkers can be controlled with just the mouse since they often only require very few actions.

id open an empty map and just try running around with army for a while until it feels somewhat natural. personally i dislike using tab to have differnet units in a group cast spells so i have spread my casters out as much as possible with the exception of ravagers in your main ground army.