Hi Pastafreak, I was worried when introducing new cats into our family, but they evenually all got on very well, just take it slow with the introductions and allow for plenty of 'sniffing' and occaisional hissy fits (sorry, teaching you to suck eggs I know).
I think it's a great idea to get your cat a friend, especially a spade female one - I would check though with the neighbour to make sure there is no medical reason as to why they are rehoming her (unexpected vets bills), good luck.