
hi shelly, we have three bullies. in general they get on really well with other members of their dog family, though i warn you that there are exceptions who are 1 dog houshold personalities (but i believe this is the case in every breed)
if you have a bitch i'd certainly make your choice a dog, as with many bull breeds they can be a bit shifty with members of the same sex unless their is a massive age gap. you will probably find that the bitch takes up the dominant role (i've had them my whole life and its always the bitches who've been boss). As to age to get one obviously everyone is different in their views. personally i feel that you should get a new pup when you think your girl is ready for one (happy with level of training etc). we have a dog and a bitch with 9 months between them and they get on wonderfully. there is potential i suppose if you leave a dog till its much older (remember 3-4 for a bully is approaching middle age, the average age is 9 years- though my last one to go to the bridge went at 14) for it to get set in its ways and not want a pup, but again this totally depends on the individual dog.
yes though they do get jealous, this has never manifested in aggression for us, just you getting bugged more often and the occasional sulk. you've just got to make sure they both get plenty of love and cuddles. you'll probably find that they have a particular favorite person but still adore everyone else (my bf is Keepers fav, I am Tios i think, etc). They will also have very different ways of interacting with different people to get the attention they want (e.g. our eldest girl has a different way of sitting with each member of the family, she'd never sit with mum the same way she does with me)
they are a brilliant breed and i;ve generally found they do better with a companion, whenever we have had 2 and one passes away they mourn terribly.
plenty out there in rescue but if your not sure about how your girl might be with a new dog maybe bring in a pup, she can then teach him his manners etc. good luck and let us know what you decide :)