re luke, ninja uses alchemy and sam uses mage, so you'll have to plan and work hard to get value from the spellcasting. it's true that even a low level of alchemy (20) will allow you to mix pickmeup potions, but the best spells for alchemy are levels 3 and up. if you wait for toxic cloud (my personal favourite), you'll need to have 15 levels as a ninja and a 60 in alchemy just to get it - and even then, when you switch back to samurai, you'll have to wait 3 more levels as an apprentice before your spell learning can continue (assuming you spent level 1 as a sam)
also, you might consider starting either sam or ninja and then replacing the character with the other, that way you get the starting equipment for both, and avoid having a nekkid ninja running around until you can buy the kit from crock
for balance, i would consider letting yoda run up to level 5 as a priest so that you can get the magic screen, armour plate, and rest all buffs. of course, this will set you back learning as a monk, but no worse than the ninja/sam described above. and you can still go staff as a priest so that you dont start from scratch when you switch to a monk.
as i recall, there are some great 2h staves for the monk, so you might not want to waste your time going dual short staves with him. on the other hand, dual nunchuck could be interesting (except you could switch to vampire chain later, but then go only one handed)... yeah, maybe 2h staff is best. (of course, martial arts is even better, but i'm assuming you want to use a weapon).
good luck and let us know how you go!