Hi,
I also have a "1tank, 3 dps, 1 healer"-setup (with 5 different classes) and I'm using a modified ftl-system (modified, because it uses an target-and-hold-mechanism, so its not targetless).

So I will try to answer your question:
Will I need to stop using shift and reserve it for the FTL system?
Well you need to use a third modifier to differentiate between 5 different possible leaders, and naturaly ctrl,alt and shift are used therefor.
But this doesn't realy mean, that you can't use alt,shift,ctrl as in your normal keybindings, because if i recal correct you are using HKN as your keybroadcaster and with HKN you can differentiate between left and right ctrl/shift/alt.
So what I do in my setup is:
- I use rctrl,rshift,ralt as the modofifers for identifying the leader in an ftl and so I can use lshift,latl,lctrl as normal modifiers in my macros.
Do many of you 5-boxers use primarily the keyboard for everything?
-I honestly never was a real 'mouseturner' but also not a 'clicker'. But scince I multibox a 1-3-1-team I do the following:
- Controll the Tank-Abilitys only with mouseclicks
- Controll the DPSers with broadcasted keys (mainly a big dps-keybind (2) and some for dots and other special abilities
- Controll the Healer via broadcasted keys (Main Tankheal) and Mouse-Over-Healing via Mousebroadcasting