Your 18 oz throttle servo will work fine. I'm using Hitech HA-55s for all of mine and they have only 14 oz. This is a good decision.

I wouldn't worry at all about the weight of sheeting to the wing tips. The weight will be minimal and should have no adverse aerodynamic effects on the plane. A lot of people sheet their wings if power won't be an issue. The 4 eggs at the end of an otherwise empty carton doesn't apply here (sorry to quibble d bodary) as the weight will be evenly distributed across the wingspan. If glued correctly, you should end up with more strength. I can't remember the wing structure of the Tracer right now. If you wish to sheet them then go for it.

Tip plates will increase roll rates as well as prevent wing tip vortices.

Remember, any plane is a collection of compromises. You certainly can reduce the compromises you don't like. It isn't difficult to improve on things that you want out of the plane. The key part is "things that you want".

For servos, maybe something in the 40 oz range would be adequate. You can find servos in this range, ball bearing maybe, digitals, etc. The choices are good.

Keep the tail light as you have a long arm compared the short distance to counterbalance things with the engine. The rule of thumb that weight to the tail compared to nose is around 4:1 would be good to keep in mind with the Tracer.