I am not sure they could actively suppress the state before mastering it, they could though indeed control the level of effort they are putting in. Trunks and Vegeta screams more of an effort thing to me, nothing necessarily related with their chi. All Yamcha's comment might mean is that Goku's chi would be perceived by them (or at least by him) as considerably higher were him putting up considerably more effort, like fighting.
#20 suggesting Goku was initially within a level that #19 could defeat and then later being impressed with the outcome of the battle could be argued under the point of view of suppression (as I think his mechanic 'scouter' would read fighting chi and resting chi as the same), but there're other ways to interpret that scene as far as I can tell, in which Gero's radar just maxing out after a certain point (due to him apparently also wrongly reading Vegeta SSJ's power) making sense as well with how off his calculations seemed to be later on. So I will just tell that whilst the Gero's part is a piece of evidence, it isn't as conclusive.
As for their surprise later on concering Goku and Gohan as FPSSJs, they just said they look pretty relaxed and resteless, natural while standing around as SSJs, which doesn't necessarily mean that was the first time they were suppressing the state. I think it could still work even with suppressing as SSJs not being a new thing for them.
So... maybe.