Hi

Well done for looking at a rescue cat.

Catflap: it depends if the cat you rescue is used to this or not. Some training may be necessary. I adopted 2 youngish adults who I had to train to go through a catflap and it took about a day with treats as an incentive. However my semi-feral much older moggie really hasn't got it, so we open the door to let her out.

(I haven't persisted with the semi-feral as she spends the night out if she wants to - she has an outhouse with a bed in it for shelter or she can stay in and wake me up to go out)

Toileting: I keep 3 litter trays in the house because at night my 2 peds are kept in. Both use these, but they also use the outside. Both prefer indoors if wet and/or cold outside.