You can unlock the golden kart in Mario Kart 7, by either obtaining at least 10,000 VR (VR is the "points" you earn playing online), or by collecting 20,000 coins (grand prix only)...
you can not at all only for mario kart 7 you can
Aero Glider:150cc all cups Star rank
Attain at least 1 Star in all 150cc Retro Cups
There is no 'best' glider or kart or anything... There are only gliders than increase your acceleration, lessen your weight... and etc. Besides, the gliders barely matter, they only change a tiny bit of your karts statistics.
Win a grand prix
Yes.
It takes 5000 coins for the gold wheels in Mario Cart seven
I don't know for sure, but I hear you get it by getting 100 streetpass tags in Mario kart 7.
by getting all gold in a cc
You can unlock the karts in any order so they are all here: Large Karts: Piranha Prowler - Win 50cc Special Cup Dragonetti - Win 150cc Lightning Cup Aero Glider - Win all gold medals in 150cc Cups
Yes it is since Mario Kart Double Dash is a Mario Kart game
You cannot unlock Baby Mario in Mario Kart DS. He is only in Mario kart: Double Dash!!! and Mario Kart Wii.