So I heard normally people can have 5 gold troops maximum, but at outpost level 19 I can have only 4 set of gold troops!
1.So increasing maximum command is the key here to make the fifth gold set?

2. If I buy gold army set from shop that can be additional set I can have? Means I have four gold set in my army, if I buy 2 gold set from shop then I can have 6 gold sets in my army?

3. I am playing human, if I buy rakan crossbow set, will my fethershot/longbow research also strengthen the rakan cross bow set I purchased from shop?

1. Hero command, army slot, duke/prince and advisor positions increases caps.
2. From what i know you can't buy shop gold units if you don't have the cap space for them.
3. No, only your current race research applies to troops you own.

You can buy any gold squad from the shop even when you have, for example, 149/150 gold cap available.
But you will go over cap if you do this, and losing gold units will mean that you will not be able to reinforce them.