At the size of these models the wing loading plays a large part in how the model flies. Sure, you can stick on a big engine and it'll fly but it'll turn ugly when it slows down. Far better to shoot for a simple model and light weight and make it a sweeter model in the air.

If you're flying off a grass field then I also recomend leaving off the gear. Especially retracts. It's very hard to make a light but strong enough retract. And any fixed gear with wheels anywhere close to scale size will need grass at least as smooth as golf green to land consistently well without numerous noseovers.

A rudder will help out if you're using the throttle a lot in flight where you want to to wingovers and snap rolls. For the usual fast and zoomy stuff at high throttle like with a non throttled engine it's not as useful. So some of this will depend on your flying style.

If you go for a .15 then using 6 to 8 gram servos in the tail would definetly be an option. And maybe for the .10 as well. If you can find a NOrvel .074 then you won't need the servos back there because it makes at least as much power as an OS 10 but at a much lighter weight. And a lot of if the servos can go in the tail will also depend on the kit's wood weight. Balsa has an extremely wide density range. Far wider than any other wood. So if it's light wood then the weight of the engine will play a far larger part in determining the CG and the servo positioning will be a big part of this. If the wood is heavy it may already be plenty tail heavy enough.

It's best if you build it up to a point where the firewall position is still mobile as well as the servos. Tape or pin it all together along with enough extra wood stock to simulate the parts not yet added and then balance it. Shift things around until you're in the ball park then build it that way. And if in doubt it's a lot easier to add a tiny bit of tail weight than a lump of nose weight. So if you have to err tend to make it a touch nose heavy during this mockup. Not a lot mind you. Maybe so it's 1/4 inch in front of where the plans say or if there's a range of position shown then at the front of the range.