I'm looking to do my first frame build out of bamboo and carbon fiber. I was hoping some people on here could give me some advice as to what are some good frames to use in the process for some of the pieces such as the head tube, bottom bracket, etc. and with it being my first build I wanted to mimic the original geometry of whatever frame I used. Will just any old steel frame work or are there other things that I should look for? Thanks for any and all help.

Any old steel frame will generally work. Take your time though. I see a lot of guys do very poor prep work of the metal before starting their lug wrapping. Don't be in too much of a hurry.
Sourcing appropriate Aluminum is also relatively easy even if you don't want to go to a supplier who provides actual frame material.
You can get stuff that's very close to the same dimensions at least to the point where someone can ream the headtube and seat tube and thread a BB shell.

I think, as you already seem to know, one of the main things to consider is the original bike's geometry. If the bike already fits you, you can replace the tubes one or two at a time and maintain the original geometry. The other thing to consider is the condition of the parts you will be using. I would check to make sure the threads are all good and if you want a derailleur hanger make sure it is already there. That's about all you need to consider if you are just choosing a donor frame.

Oh, I came on here looking for info on bamboo frames. I really want to try and build one myself (although if someone dropped a Boo bike off at my place I would probably not build one.) Great advice on replacing one tube at a time... I was wondering how it would all work if measurements were off by a little. Maybe I'll try just the top tube first and then as I get more confident move around. I want to build a bamboo cross bike, so I could even leave the canti posts on and just replace the stays below the posts!

I am going to check back here often for further advice. I got myself all excited about a bamboo frame today!

I've always had a bamboo bike on the back of my mind.

I recently learned that you can make oil soaked wooden plain bearings by boiling hardwood in oil. Bamboo ranks pretty highly in hardness. I wonder if bamboo hubs and wooden bottom bracket are possible. Bamboo rims are probably also doable, and bamboo handlebars are certainly feasible.