The future polish is very thin, almost like water. I usually squirt some in a disposable cup and put it on with a throw away foam brush. Clean up with soap and water. It dries quickly...not more than 30 minutes or so. A bottle will last you a very long time. I've never had any compatibility problems with it...with various brands of paint and various decals. It's used to coat floors so it's also very tough. You can search this board to find numberous tips/hints on Future polish.
I've never used the Testor's gloss coat, so I can't comment on that. The Dull Coat does work good though. I've even heard of guys putting some talcum powder into the Future polish to get a dull finish. Never tried that though.
Good Luck,
John