I prefer to have one aileron servo for both ailerons up to about 60 size. The servo only needs to be powerful enough to roll the wings so there isn't much stress on it. I also like bellcranks and haven't had problems with them. These days it's fashionable to put an aileron servo in each wing half. Do whatever blows your skirt up.
As far as modifying plans, the skys the limit. I modify nearly every plane I build just a little.
Here's how I do wing ribs: Trace the rib over the plans using tracing paper. Cut this out leaving a little paper outside the lines. Lay the paper down on the garage floor and very lightly (LIGHTLY!)spray it with 3M 77 spray. Next, invert the can and spray to let it clear out so the nozzle doesn't clog. Place the paper template over the wood piece that will be a rib and rub it down with your finger. Now stack as many pieces of balsa as you need ribs on top of one another and tape them together with masking tape in several areas. Cut out your ribs on the bandsaw being sure to retape areas that were just cut to hold the stack together. After you've cut all around, check to make sure you have a nice fit for your spars then remove the remaining tape and admire your work. The top rib template you glued down should remove easily but don't let it stay on the wood for long. Note that I said to spray the 3M77 on the template, not the wood.
As an alternative, you can use double sided carpet tape to hold the wood pieces together but my experience is that type of tape is really really really really hard to remove and you will wind up cussing it and possibly breaking a rib or two trying to remove it. When I don't have a better alternative, I cut that carpet tape up into tiny pieces so it will be easier to remove. But I still don't like doing that.
Good luck. Building from plans is very rewarding.
carl