It's definately worth a try. When we first got our cat Patches, or other cat Kess (who is spayed and older) hated her. Kess peed all over the apartment, Patches was about six months so she would play and just tick her off. They stayed like that for several months, mainly just avoiding each other.

Now a year later they snuggle on couches, they eat together and play together. At first they couldn't be in the same room. I definately recommend closing your bedroom door, it could get messy! We figure it was just an introductory thing. Hopefully they will warm up to each other. Depends on how long its been already, and how stubborn they are! Good luck

I think it is worth a try, too. I adopted two 10 month old siblings when my old girl was 15. The old girl never warmed up to the young ones, but eventually they all learned to tolerate (even, on occasion, groom) each other. While one of the twins, the neutered male, often tried to play with the old cat--pounced, chased, etc to her great disdain--he also played and wrestled with his twin sister, so having another young cat did greatly diffuse his energy.

I think it's worth a try, too, esp. since you can return Clarence if things don't work out. If Simon has a cooperative and appreciative play buddy, it might just take the pressure off of Del. As long as you're careful not to allow the boys to gang up on Del, and as long as you keep a very close eye on Del to make sure she doesn't come down with a stress-induced illness (like UTI), things might just work out.

Be aware, however, that if Clarence does induce Del to start peeing around your place again, that behavior may not subside just by sending Clarence back to the foster home. Depending on how stressed and angry Del becomes, inappropriate urination could be a behavior you're going to be dealing with for a long time to come.

Good luck!

Laurie

Actually, we did this with very good results. When our 15 year old cat died of cancer, we got a male kitten. He was a rather playful with our 13 year old cat, who tolerated him better than I thought she would, but he wanted to wrestle and she clearly didn't. The kitten turned into a 20 pound adult male and he still wanted to wrestle.
So I got another male kitten and all is happy now. The two males wrestle each other and mostly leave the old cat alone.
p.s. For the first few days, the old cat was more welcoming of the new kitten than the 20 lb male, who must have felt his territory was being invaded. Now the boys sleep together in a pile.

Thanks, everyone. Well, I didn't think I was looking for a third cat, but what could be better than THREE brown tabbies? I'll give it a few days or a week to think about it before I decide on a trial.

Del's been and on again/off again pee-er her whole life. Every new boyfriend/room-mate/visiting animal, etc. has set her off. I did find out very recently that she's in the beginning stages of hyperthyroidism. Could the stress of Simon have caused that?

Good point, Laurie, about the possibility of continued peeing even if Clarence goes away, and about the ganging up. All indications are that that's not Clarence's style, but I've learned that sometimes a cat in a new situation can almost become a new cat. It's been over a year since I got Simon, and, well, things have improved, but there's still some conflict and tension.