Some suggestions that you may not accept

[BShen-V12]

#1 yes.
#2 yes.
#3 yes.

#4 I have a special idea for this: Level system? Which means the more RP you got(as in level), you get more slots. Limited, though.
I suppose 8 slots as max (for each available class - with one possible "muscle" class in future) or 10.

I'll explain that level system here. 
Level 0 - 0 RP
Level 10 - 30000 RP - Unlocks slot #6
Level 20 - 120000 RP - Unlocks slot #7
Level 30 - 300000 RP - Unlocks slot #8
Level 40 - 600000 RP - Unlocks slot #9 (if available)
Level 50 (MAX) - 1000000 RP - Unlocks slot #10 (if available)

not accurate ones though, just my suggestion. final results may vary.

#5 yes.