im just wondering why you cant put the guinea pig in with the rabbit while it is the holidays ,
they should by all accounts get on very well with each other (unless your rabbits ae breeding?? )
this would save a lot of building and a lot of money as they can share the same run which by all accounts sounds a very very good sized one
if you cant for some reason put the guinea pig in with the rabbits i would recommend a 4 ft by 3 ft section as this is a good size
you can have them smaller than that but if you want to give the pig the space to play and explore then 4 by 3 would be the best
best of luck with your rabbits and your guinea pigs

and im sure no matter how big the run is the pig will be happy

especially if he has grass coming from the bottom so he can play in it
katie