I'll answer the last part... I've never heard of a plane where the CG should be adjusted for a wet condition (with fuel). Also, 20oz is a large tank for a 26cc gas plane. I'd think 12oz would be more appropriate and I say that from a safety perspective. If something happens to throttle and the plane has to be flown until out of fuel before landing... 20 oz of fuel is going to be a very long time. Probably 30-45 minutes and if at lower throttle setting, perhaps even an hour and if it is on a later flight of the day, battery life or daylight might be an issue.

There is a long running and current thread on the Seagull Yak so a search of it might produce your answer or a post there might catch the attention of the guys flying that plane.