Cannondale Trail SL 3 component upgrades I have a 2013 cannondale trail SL 3 and some parts have been worn and look beat up so I'm planning to replace them. I was wondering if I can use a 10-speed rear derailleur on a 9 speed cassette and can I use any shimano shifters as long as they're 3x9? (I.e. Shimano Deore shifters on XT front and rear mech all 3x9. Also, can I change the crankset as long as it has the same number of teeth on all chain rings? Thanks

Specs:
Fork    RockShox XC 30 TK, Coil, 100mm

Hubs    Cannondale C4 front and rear

Cranks  Shimano FC-M430-8, Octalink, 44/32/22

Bottom Bracket  Shimano BB-ES25, Octalink

Chain   Shimano HG53

Rear Cogs   Sunrace M96, 11-34, 9-speed

Front Derailleur    Shimano Acera, 31.8 clamp

Rear Derailleur Shimano Deore, shadow design, 9-speed

Shifters    Shimano Alivio

Brakes  Shimano BR-M446, 160/160mm

Brake Levers    Shimano BR-M446
My understanding is that 5 speed is unique, 6/7/8 all share the same chain specs (unless its weird) and that 9 and 10 and 11 are all unique again.

You're probably best to keep everything in the 9 speed range, and perhaps enjoy some tech trickledown from the higher grades.  

Your specific questions


A 10 speed deraileryeryer will probably work "okay" but the wire pull is different, so in the middle of the cassette should be fine, but as you approach the outsides it will progressively not go quite far enough.
You can use any 9 speed shimano shifter on a shimano 9 speed setup.
The crankset does not need the same number of teeth, but if you make it too big or too small, then the front mech may be too small to span all the chainrings.


2013 is not that long ago - for a bike part to be worn out in a couple years suggests that you're using it well, or mistreating it.   Care for your ride and it should last 10-20 years.