We use the nylock "aircraft" nuts for mounting the engines all the time. I highly reccommend them for prevention of the engine from comming off. don't skip the flat washers though...

The flexibilty of nyrod on a nosewheel steering linkage is not a big deal. It actually helps prevent damage to the servo gears when you hit bumps... though it can alow the nowewheel to turn a bit (usually in the direction that would be "pull" for the linkage.) the turn is usually so small as to be unnoticeable... if its large enough to notice... it probably saved you a servo gear set.