If he's biddable and can be checked in when needed...Go a head ,they can put birds out where the bigger dogs can't...the runs in the Gorse will be a pisstake to him..
If you have no confidance on his control....defeats the purpose a Tad ! nothing worse than climbing over Treefall,navigating woods ect.. with a Terrier straining at the leash ..Wanting to Join In !
Try and get a "DRY run " if possible on the ground your likely to be on..Get to know the Ground ,if you don't allready Know..Lot of folk Beat ,but don't take in the positions of earths ,likely Distractions for a Terriers . Rabbits,sh