I almost always use my bard as the desginated haste guy since it only cost stamina and I can cast in at the beginning of every battle.

I have used a bishop as the haste guy once, but he was disappointing coz he ran out of mana too fast from casting lvl 7 haste at the beginning of every battle. (I am not a fan of resting all the time). Well anyway when the bishop didn't cast haste, my paRTY WOULD GET fewer swings and miss out on valuable practice.

I wanna try a 4-man party...but in my proposed 4 man party (Faerie ninja, monk, bishop and rouge)...there is no room for a bard. I was thinking if I use both the bishop and the monk for casting haste, would that be sufficient...

P.S. Again it would help me greatly if someone can tell me the earliest parts of the game when I can get some SP regen items.

If you start your Rogue as a Bard and switch at L8, you'll have no interruption to your Locks&Traps development, your fighting progression will also continue, and you can keep using the instruments up to L8 as any other class. For this, i recommend practising with the Charm instrument until you have at least 75, because you'll almost never fizzle. You just won't be very good at attack music.

But isn't powerfulness of the rogue's backstab dependent on the rogue levels? So you would lose 8 levels of backstab damage progression, right?