I think you are stuck getting 2, to make sure the pieces are of similar size and shape and allowing you to control the cooking. I prefer getting untrimmed so that the trimmed bits can be stroganoff, filet mignon, etc.
When I have made whole wellington, I find it easier to spread the paté and duxelles on the puff pastry and then put the beef on that and cover with the sides and ends. It's not a complete circle of paté and duxelles but a little easier to do than spreading the beef. If will be tricky to par-cook them differently and then try to get med rare and med well but, hey, cooking is nothing if not shooting from the hip.
I have made individual wellingtons and it's not that bad, more of an assembly job. You can cook each round of filet to desired doneness and it can cut down on the amount of meat you need because each piece has a bit more pastry (better to soak up a red wine sauce?). Also, I found it easier to control the degree of doneness, marking the more well-done pieces with some kind of a pastry cut out decoration.
I have made lots of chicken and to be honest, the best response always come from a well-tended, beautiful roast chicken. It's not particularly difficult but requires your attention.
After that, I think it is poached chicken breast (in stock, wine, flavorings) and then a finished classic velouté or variation. I made sauce allemande the other night and we couldn't get enough.
Cheers! Let us know how it turns out.