Hope you've got a sit down mower to do the lawn!!!
Guinea pigs do the same job
The garden looks fab, they will love it! I would keep in at least month and let out on a day your off work so you can keep the door open. And do it on an empty stomach! (the cats not you lol)
I would shut in the one room overnight if theyl stay if not then let them look at their own pace. All depends on their characters really, i know my lot would vanish and not be found again!
Most important is you keep the windows shut.
Are they chipped? I would reccommend you get them done if not. If they are, dont forget to update your address with the chip company! Nothing more annoying then a lost cat with chip whose owners have moved