do i need to use dowels if i am using a 2 layer 6 in and 2 layer 9 inch. if i do then would the thicker bamboo skewers work?? how many would i need???

you definitely need support. Look into the SPS system. I will never use dowels again. Had a near disaster once.

I would definitely put something in there for support, probably 4 supports.  For that size cake I like to use the bubble tea straws - much easier to cut to size and insert and a 6" cake isn't heavy enough to where you have to worry about them not being strong enough. 

AYou always need support for stacked cakes. Make sure you have cardboard cake rounds (foamcore, sep plate, etc) between each tier as well as some sort of dowell.

I'm not sure how big bamboo skewers are but I don't care for wooden anything. I like poly dowells or sps. To each their own.

AWhat is an SPS system and bubble straws?

Ahttp://www.globalsugarart.com/bakery-crafts-pillars-plates-sps-system-c-886_887.html

Plates with legs that you cut and slide into your cake. Bubble tea straws are thick milkshake type straws

AThank you for the link. I do have other questions about the SPS system. You need the pillars and the plates for it to work correctly, right? Also is that something that gets returned from the customer or is it just part of the cost of the cake. I am fairly new to all this so please excuse any dumb questions.

AYes you want the plates and the pillars, they'll click together. You'll cut them to fit the length of the tier or higher if you want a gap. I have been able to reuse it he plates they're plastic so you can wash them. If its a far delivery I don't ask for them back but if it's local people always end up returning. I don't use sps for all my cakes, but I always factor in supports and boards and boxes.

All stacked cakes need support, I like plastic dowls