can i compress the tire on my freestyle or only on trials? 
Well, there’s not really a whole lot there to compress. Trials tires give you more volume for more compression.
Yes, but less.
I can get enough of a prehop on my freestyle unicycle with a 2 x 1.95in tire filled to about 40 psi for it to help. However, there is a smaller margin of error to prevent flats or maybe potentially roll the tire off the rim, and the spring effect is not as powerful as with a larger tire. Also be aware that many freestyle unicycles are built with parts not strong enough to handle the stresses of prehops.
The thinner the tyre, the higher the pressure you need. The higher the pressure, the harder it is to compress.
My 23mm tyre runs at around 120 psi - 130 psi which is almsot solid. My fat muni tyre runs soft enough to squeeze a bit between finger and thumb.