I learned the hard way, you are stuck with a single-zone cymbal... period. I picked up a 3-zone 14" Ride (the one with the two input) thinking I could run the ride in one input and then assign the bell to the other)... it doesn't work that way. There is only a single piezo in there. The other two zones use what I think are called "film switches" (similar to what keyboards use) that simple get physically pushed together to make contact. Unfortunately, the Nitro Mesh does not recognize these types of switches.
SO, you could pick up just about any 14" or 16" cymbal as long as it uses a piezo and use it.
As for the snare drum, I picked up a 10" Surge snare. It works great. I assume you could use just about any Alesis snare with the Nitro (up to two-zones). Your biggest issue is going to be the mounting system. The Nitro Mesh kit's mounting system was ... ahem ... modified... slightly... in order to make it a pain-in-the-backside for the entry level kit to be upgraded with superior parts (I imagine... this is purely conjecture on my part). What you will find out if you pick up anything other than a Nitro snare is that while the Nitro mounts on to the 90 degree angle par HORIZONTALLY... just about everything else mounts VERTICALLY. No big deal... you just have to spin the mount 90 degrees... as long as you use your current mount. If you try to use the mount that comes with most of these snares ... you'll find the piping used for the rack is of a smaller dimension. One way to "overcome" this problem is to use a piece of cardboard wrapped around the bar to increase it's diameter. It's not really that hard, but I found it strange that Alesis would alter the mount in such a way.
Of course you could always use a snare stand and any/all mounting issues are moot.
Things to keep in mind for expansion with the Nitro...
ALL Tom locations are 2-zone capable. The Nitro Kit comes with a single dual-zone snare... the toms are all single zone... but you can stick that snare at any tom location and maintain dual zone.
Cymbal locations are all single zone choke-capable... period. This includes the expansion cymbal location.
Basically, in addition to the standard Nitro-Mesh kit you have expansion for 6 additional pads/triggers in total. 3 additional triggers at the 3 TOM locations that come with the kit. 2 additional triggers at the 4th Tom expansion port... and 1 additional trigger (with choke capability) at the 2nd crash cymbal expansion port.
Each of these trigger locations can be used for whatever you want. I currently have Tom #2 a dual-zone (my original 8" snare)... then using a stereo to 2-separate mono channel splitter, I run one mono line to Tom 1, the 2nd mono line to my custom bell trigger. I run one mono line to Tom 3 and the 2nd mono line to another custom bell trigger.... and tonight I will be running a 3rd splitter and running Tom 4 to the Tom then the other line to the 4" "bell" cymbal custom trigger I am making to stack on top of my current ride.
I will then be all triggered out... being I already have a 2nd crash.