Take the amount of grain, extract, hops, etc necessary for a 5 gallon batch and divide by 5. The gravity will be the same as for the larger batch. As long as the gravity is the same so should be the taste and alcohol content. Pitching a full vial of yeast into that small a batch of wort will get fermentation going sooner and will shorten the fermentation time somewhat. You are a lot more cautious than I am. I tend to take a given beer and tweak it a little at a time in 5 gallon batches. As long as I don't go crazy the result will be a good drinkable beer.
I did do a batch of Scottish Ale where I increased the amount of peated grain by 0.5 oz and discovered I had a really meaty tasting ale! One of my friends called it "the best bar-b-que I ever drank". Fortunately, the smoke flavor decreases with time and after about a month all was well. The next time I only increased the smoked barley by 0.25 oz and got to where I wanted to be without the extra wait.
I mentioned my mistake for a reason. Experimentation is fun but you are best served by taking a known recipe and tweaking it a little at a time rather than making bold strokes. A little research beats striking off into the unknown, also called the ughknown around here. That way you can avoid having friends who say "Well, it's...different". LOL!
Just my 2 cents.
John