Hi BF/Mechanics. I poked around the search function but didn't find an answer to my particular question.

I'm swapping out part of my drive train today to accommodate a bigger cassette (for touring). Currently running a standard 10 speed road setup with friction shifters.

The plan is to go with a Deore long cage derailleur to accommodate a 9-speed cassette (that has a larger big sprocket)

I want to stick with my current 50/34 road cranks and front derailleur. Keeping in mind that I'm using friction shifters, am I going to run into any problems with the 9speed rear and 10speed front?

My apologies if this is a stupid question. I just want to make sure before I spend a few hours doing it up, only to find out it won't work and be unable to return now-greasy parts.

I assume you plan to use a new chain with the 9-speed cassette, both for adequate chain length and to ensure they've got the same wear history so they don't skip. If it were me, I'd use a 10-speed chain with the 9-speed cassette here. When using the small chainring, if you shift out far enough on the cassette, the chain's approaching the crank from enough of an angle to start grazing the big chainring. This is an increased factor with compact doubles, since the ratio of chainring sizes is pretty large. The narrower 10sp chain will probably allow you more gears before that starts happening, and works fine on 9sp cassettes.

Yargh! Really? I was trying to avoid the 10speed chain cause they're so bloody expensive.

EDIT: I currently have a few mms of clearance between the chain and the big ring when I'm in the small ring and the smallest cog. Will a 9 speed chain add that much thickness?

You have a point about the cost of 10sp chains. If cost is a factor and clearances look OK, sure, go with a 9sp chain. Worst-case scenario, you might have to avoid using a cog or two, easily addressed with a double-shift to the big ring and a lower cog in back.

I'm all confused here. What is 9 speed and what is 10 speed? Are the crankset chainrings 9 speed or 10 speed? Is the chain already 9 speed?

9spd = 9 cogs on the hub. Cranksets, chains and front derailleurs are referred to as 9 or 10spd because these chains are narrower than the old standard width used by 5-8spd, and so chainrings needed to be put closer together, and FD cages made narrower.