Sure, you can do that. Don't halve the yeast, though- you can go ahead and pitch the whole thing of yeast.
One thing to keep in mind- you'll be doing full boils (since your batch is only 2.5 gallons) and you'll get better hops utilization. You may get a little more bitterness out of your hops- not necessarily a bad thing, just be aware of it. Also, since you won't be topping up with cold water, make sure you have a way to cool your wort quickly. I would just use an ice bath in the sink. You may want to get a 3 gallon carboy for your secondary- I have a couple (one glass, one better bottle) but the same size primary is ok.