i have been researching my 5 breeds for 5 years now, and will soon have the chance to own one of each.

I was wondering whats the minimum age you would suggest getting a second pup?
i was thinking around 5 months between each.

At five months they are still very much pups and still require a lot of training and input.
So in effect you would have two pups at the same time - hard work i would have thought

It depends very much on how much work is put in with each puppy. I wouldn't consider getting a second puppy until the first is fully housetrained and reasonably well-trained in all other respects. Probably about a year between each would work - the gap getting longer the more dogs you have.

I waited untill buster was nearly 2 and that he was well trained so i could cope with another dog. Although hes learnt all the bad habits of bengi (new addidition / rescue) and only taught a few good ones to him. :p

I would say a minimum of two years, and ideally nearer 3.  By this time you have a fully trained dog who will set a good example for the new pup, and will be respected by the pup as an adult.  So you should have your fifth dog when the oldest is between 10 and fifteen years old, by which time you will be going onto number six as you loose the first one to old age.

I have five dogs myself, oldest is 12, next one 9 1/2, then one nearly 7, then 4 1/2, and youngest 9 months.

Dogs pick all the worst habits from each other and none of the good, sadly. 
I would wait until your first one is at the very least 18 months and well-trained.  

5 months is only 20 ish weeks - they will both be puppies up till about 1 year and teenagers thereafter!