Does the Supercarb W9122021 fit a standard 10" filter housing?

Does the Doulton Supercarb ceramic filter W9122021 fit a standard 10" filter housing?

No, that one fits in Doulton filters only but Doulton Supercarb W922901 fits most industry standard 10" filters which can be found on the following link:
http://www.doultonusa.com/HTML%20pages/ceramic_cartridges.htm#DOE

Inline undersink water filter

I am interested in having one of your Doulton ceramic systems installed in my kitchen undersink. I'd like it to be an in-line system so I don't have to have an additional tap on my countertop. Is this possible?

We receive same request as yours every now and then which we recommend our Regent I which have high capacity and flow rate with Imperial Ultracarb Cartridge It is only available from us and the cost is $249.00 without the faucet and install hardware. For more details visit the following link: http://www.doultonusa.com/doulton_water_filters/Imperial-high-flowrate-RegentI.htm

What install hardware do I need with this setup? also, does this system provide maximum filtration?

Your cold water shut-off valve is most likely located in the center back of the sink base cabinet, You want to install the filter closer to the cabinet door as you may need to clean the ceramic cartridge every now and then as well periodic change out.
We can give you 3-4 feet of 3/8" PE tubing for the filter supply. On the shut-off valve end you'll need a 3/8" plastic compression tube ring (delrin) and a 3/8" compression nut (use the existing if you can) to attache the filter supply and the other end of the supply simply push in into the filter inlet (build in push-fit inlet/outlet).
For the filter outlet to the faucet you'll need standard 3/8" faucet riser of appropriate length (gray plastic riser). All the items you need are commonly found at local "big box" or hardware stores. Should you need any elbows to avoid sharp bends (kinks) we can supply you with push-fit elbow (s) at no charge. Push-fit connection instructions are supplied withe the filter. The Imperial Ultracarb filter efficiency is same as with our slimline Ultracarb element.
Push-fit instructions can be found on the following link: http://www.doultonusa.com/Instructions.htm

Looking to replace bottled water service with inline water filtration system.

We are a business of about 50 employees looking to replace bottled water service with inline water filtration system.

In looking at your website it looks like what I had in mind is something like:

Under the sink Chiller® with Doulton filtered water: The Chiller water system UC-2 has 7 gals./28 L per day chilling capacity.

Knowing nothing about this subject, I am wondering if our goal of chilled, filtered water would be met by this system or if you have any other recommendations, suggestions?? Our employee break room has a fairly standard kitchen sink, plumbing etc. and we have minimal counter space so under the counter is an attractive option.

Your best item for your purpose will be a system that we offer on our "Specials" page which consist of Regent I high capacity commercial filter, UC-2 chiller, dual temperature (chilled and ambient) faucet and complete installation kit for $799.00. The filter Imperial Ultracarb cartridge capacity is 2000 gallons to maintain premium quality drinking water (better than any bottled water). That is about 400 five gallons bottles and the replacement cartridge cost $115.10.
This package can be found on the following link: http://www.doultonusa.com/HTML%20pages/saving.htm

Many employers also offer combination, instant hot and chilled filtered water which are installed in under-sink cabinet. These can be found on the following link: http://www.doultonusa.com/Luxury-Plumbing-Fixtures/mt550_instant_hot_water_dispenser.htm

You had previously advised that the Regent I was probably our best bet. I was hoping there was someone out here that knew your product and could handle the verification of fit, installation, etc.
I could order this but do not want to be the install guy. Suggestions?

Installation is very simple and is designed to be installed by average homeowner knowing how to use players and screwdriver. Everything is included for complete install including instructions. All connections are push-fit http://www.doultonusa.com/Instructions.htm
Furthermore we'll be glad to help over the phone if the installer need any help.

The CU1200 is Doulton Ultracarb candle part number W9123053

I need some replacement candles for an undersink stainless steel water filter and wanted to be sure to order the right one for the model I have. I believe the replacement filter is for the model # IS100UC, which would make it a CU1200? I do not have the unit in front of me but I purchased it several years ago. Also, I would like to order the replacemnt 'O' ring for the stainless steel canister if available.

The CU1200 is Doulton Ultracarb candle part number W9123053 and is readily available as well gasket for your filter.

You can place your order on the following link for the Ultracarb candle http://www.doultonusa.com/HTML%20pages/ceramic_candles.htm or by phone