To answer your first question,
Does this mean there are other colonies with people like those from the Reiss family who can control titans?
It would be enough to state that you don't need to be a member of the royal family to use the power of the Coordinate. You need to be a member of the royal family to use the coordinate's power optimally.
The reference being in chapter no. 50, Eren, while still in human form, used the power of the Coordinate to order titans to eat the Smiling Titan and attack the Armoured Titan.
In fact, any one can control the titans, as done by Eren, the royal family, using the coordinate, or by Zeke (beast titan), his method remaining unknown yet.
So, no, there doesn't need to be a colony with other people who know how to control the titans. But I won't be surprised if the mangaka came up with one.
For your second question,
Can we conclude that there are other titan shifters who fight against humanity?
We can say that there are certainly many titans fighting against humanity, many of them have a certain level of intelligence, as shown by the quadruped titan carrying the goods, informing Zeke, Reiner, and Bertolt of the Survey Corp's advance in chapter 77.
As of now, there isn't enough information to definitely end the question by saying that there are no more titan shifters, the reason being that they can create one at any moment by devouring one of the existent titan shifters. The list of known titan shifters can be seen here.
So, as of now, I shall say that there isn't any other titan shifter shown/mentioned in the manga, other than those mentioned in the list.
Edit 1: According to the latest chapters, chp nos. 86-89 to be precise, there can be a total of 9 Titan shifters with special powers, whose powers can be transferred from one person to another by consuming their nervous system. Out of those 9 powers (8 people), 6 people fight against humanity. The two people in humanity's favor are Eren (Attack Titan, Founding Titan) and Armin (Colossal Titan).