There were six ponies, but seven Changelings, as the last Changeling was disguised as Spike, So Spike volunteer to watch over this last Changeling himself.  Twilight thought about protesting to this decision at first, because she thought he might not able to handle it.  But since he is the one who be had befriended Thorax first in the past, it was actually fitting that he would be the one who fit this task the most.  So with optimism, she let Spike watch over the last Changeling, and since Spike stayed with her in the same castle, she would still be able to watch over the both of them anyway.
The last Changeling was quite small, the same size as him.  But it said that it was already an adult, explaining that this was as large of a body it could grow.  That is why it was perfect to disguise as Spike, because the Changeling couldn’t disguise as something that was too big or too small from its original size.  Spike wondered if there were any more reasons why it chose to disguise as him…
The last Changeling said that because it's a sidekick just like Spike, (and the kick part is completely true), it is like the omega of Changeling's pack.  The weakest and smallest Changeling in this team of infiltrators, it was someone to kick around when the team was in a bad mood.  The joker of the team to make them laugh, easy to ignore and be forgotten... The only reason the team needed to keep it, was because it was the only one that could disguise itself as a filly or colt.
The last Changeling disguised itself as a Changeling version of him.  But it seems that it preferred a female version of him.  It said that it just wanted to be in a female shape like all the other Changelings in the team.  The last Changeling happened to be very obedient and helped Spike with every chore the he had in Friendship castle.  His chores was done so fast with its help, so he had a lot of free time left each day from that point on.
Spike was always very grateful to have this Changeling as his work partner.  So he treated it well and hung out with it after their chores were done.  He introduced it to many funny games and movies.  Taught it how to read so both of them can enjoy comic time together, and also found out later about this Changeling’s love drawing.  It told him that it had drawn a funny picture on Fluttershy's throne before when it was disguise as him.  Sadly, it was kicked around by the others after doing that...
One day, the last Changeling smiled and said thank you to Spike with a happy voice.  It said that this is the first time that someone noticed it, cared for it and made it happy.  He was the first true friend that it ever had and the love he let it feed on freely was really delicious.  It admitted that at first, it thought that Spike was a worthless sidekick, just like itself.  But no... Spike wasn’t a worthless sidekick.  He was an assistant, a supporter of the mane6, and it wished to be like him.  It wished it could be more useful for its own team, it wished that someday they will notice it and see it as a friend on the same level as them.
Spike said Changeling can change, and explains that the other Changelings seem to be starting to change as well.  They are less aggressive and friendlier now, after spending their time with each of the mane6.  He believes that they will see it as friends now and he also said, that if it really wanted to be the perfect assistant, he will help train it, because he is the number one assistant to the Princess Of Friendship after all.
So Spike took his Changeling as his student, just like Twilight took her own Changeling in the end