Second dog help......?

I want to get a Second dog preferably a female german shepherd. I have a male GSD who is about 15months and extremely aggressive with other dogs while very friendly with humans. I'll only buy a 4 month or older gsd otherwise i cant manage her by feeding 5 times a day or so. I'm also doing my 10th Standard and my mom&dad are working. Is it advisable to buy it now or later? & if i don't buy a gsd what other breeds are good for guarding? i don't want a rottie because she'll be too big & i know it because my uncle has a rottie. Please help.