I do that all the time for the same reason. Today I converted Jamil's Oatmeal Stout into a 2.5 gallon recipe and have it sitting in the primary right now. I just use a standard 6.5 gallon bucket as a primary and a 3 gallon glass carboy I got for free as a secondary.
I used the WLP013 only because I had an ESB in the primary and tossed this onto that cake. I would normally use Safale S-04. I did have a bit of a problem with this and ended up with 4 gallons or runnings and had to do a 90 minute boil to get it down to the volume I wanted.
The secret is to use good brewing software. Then you can pump down the grain bill and adjust the hops to get to the correct IBU. Sometimes I'll fudge the numbers a bit so I can get to a nice even grain bill. I do not use the scaling option in the software. It ends up making bills of strange amounts.
BeerSmith Recipe Printout -
www.beersmith.com
Recipe: Jamil's Oatmeal Stout
Style: Oatmeal Stout
TYPE: All Grain
Taste: (35.0)
Recipe Specifications
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Batch Size: 2.50 gal
Boil Size: 3.61 gal
Estimated OG: 1.056 SG
Estimated Color: 35.3 SRM
Estimated IBU: 35.8 IBU
Brewhouse Efficiency: 62.00 %
Boil Time: 60 Minutes
Ingredients:
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Amount Item Type % or IBU
4.75 lb Pale Malt (2 Row) UK (3.0 SRM) Grain 72.96 %
0.50 lb Oats, Flaked (1.0 SRM) Grain 7.68 %
0.38 lb Chocolate Malt (350.0 SRM) Grain 5.84 %
0.38 lb Victory Malt (25.0 SRM) Grain 5.84 %
0.25 lb Caramel/Crystal Malt - 80L (80.0 SRM) Grain 3.84 %
0.25 lb Roasted Barley (300.0 SRM) Grain 3.84 %
0.80 oz Goldings, East Kent [5.00 %] (60 min) Hops 35.8 IBU
1 Pkgs London Ale (White Labs #WLP013) Yeast-Ale
Mash Schedule: Single Infusion, Medium Body, Batch Sparge
Total Grain Weight: 6.51 lb
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Single Infusion, Medium Body, Batch Sparge
Step Time Name Description Step Temp
60 min Mash In Add 8.46 qt of water at 165.5 F 154.0 F