help me with this cat

[Elderlycatwoes]

We have recently inherited a cat from a family member who unfortunately died and there is no one else to take her. She is a lovely cat but is very elderly (at least 20) and has only previously lived with the family relative who had no other pets. We have 2 other cats and a dog which hates other cats.
We have been keeping her isolated in the lounge with the occasional outside visit in the garden (completely enclosed and she is way too old to climb) she has met the other cats and dog briefly. I think the cats while slightly perturbed will be ok but the dog sees poor cat as a trespasser and has been going nuts barking (only seen each other through doors so far and one fleeting face to face which left the cat trembling) Our dog is big and could do some serious damage to her so I am terrified about how to get to approach this So first problem is how do I overcome this issue
Second issue is around the litter tray. Whilst the cat uses the litter tray she often misses and ends up pooing outside the tray. She also seems to have very runny poos . We have blankets down to protect the flooring and have got an extra large litter tray but this is really getting me down I am constantly clearing up mess and the washing machine is always on cleaning blankets . The only space we can put her is the lounge which used to be my evening haven . It is no longer relaxing with the litter tray in the corner and she always seems to use it late evening and invariably misses (tonight she managed to poo up the wall!) so I tend to start and finish my day cleaning poo mess .
In all honesty it is really getting me down . Any help would be greatly appreciated . We have had her 3 weeks now with little progress

Can she get in and out the tray easily, it's not too high? You could use disposable puppy pads around it to save on the cleaning

Thank you for taking on this poor old kitty

can you get the cat and dog use to each other by rubbing each one with a bit of cloth ( sock ?) and put it the other ones area , then near , then in their bed . near food etc . then put a baby gate between them so they can see / hear / smell each other , stroke cat let dog sniff your hand and vice versa .
Once they seem to accept each other , no barking / cat relaxed introduce them while your dog is on the lead .
I think it will take a few more weeks yet but as your dog is use to cats I think it will work out

Missing the litter tray is a bit easer to solve , get a covered tray so she cant poo over the edge , I had to do this with my cat as he always stood on the edge of the tray and tipped it up , litter and poo everywhere !
Use it with the door off to start with until she gets use to it .

The runny poo , are you feeing her the same food / amount / time as your family member did ?
A trip to the vets may be needed to give her a once over .

Thanks for replying . She seems to get in ok because it is quite low I think the main issue is she cant squat down properly I think . Her back legs only go down a little bit so her bottom is still above the top of the litter tray and as a result she is pooing horizontally instead of downwards so poo falls outside the tray onto the blankets or onto the wall. But any higher walls around it would mean she probably won’t be able to get in

@DeanStockwelll thanks for your reply . I will try the smell thing from tomorrow and see if we can make progress with the dog .
I thought about getting a covered litter tray but wasn’t sure if she would recognise it as her litter tray as she is used to the standard open one but I am pretty desperate now so will give it a go .
Food wise we did inherit some food as well but not much and she didn’t seem keen on it although this may have just been stress as she was pretty withdrawn when she arrived . So we switched to other types. The last couple of days we have switched to dry food and fresh chicken so no wet food but still no change. I might try the inherited kibble again and see if we have more success as still have some left . She is a good eater on food she likes but seems quite fussy but drinks lots of water .
The loose stools have been pretty much consistent since she arrived . I wasn’t sure if it was an age thing for cats

I’d get her checked at a vets , when my mums cat had loose stools it was a thyroid issue .

With the litter tray , put about half of her used litter and some poo in the new tray and once she has used it a few times clean it out a bit but always leave a bit of used litter for the 1st few weeks .
When you do eventually clean it out completely don't used anything scented to wash the tray with .
Always use the same brand of litter.

With her food it sound like you have ( with the best intentions) swapped round quite a few times . Go back to the one she has been given by her previous owner and feed small portions several times a day rather than one or two larger meals.
An elderly cat probably won't have a very big appetite anyway so we'll eat as much as your other cats.
Make sure its away from the other cats and dogs as most cats fell quite vulnerable whilst eating so she needs her own space if you can.
It's definitely still worth taking her to the vets give her a check over.
Good luck

Thanks for the replies. I will definately try the food we got again and take her for a check up at the vet. DH did call round at the vetlast week just to check we weren’t missing any medication as don’t know her history but with the loose stool’s it is worth getting her checked over again