For your first question,
Does this mean there are other colonies with people like those from the Reiss family who can control titans? The conversation you showed only implies that there is another colony (or colonies) outside of the Walls that aims to recover the Founding Titan. It does not immediately imply that said colony (or colonies) can control titans, only that they know of the Founding Titan and possibly its abilities.
Contrary to an earlier answer here, you need to be of royal blood to use the true power of the Founding Titan. In Chapter 106, it was mentioned that Eren was only able to use the Titan Control when his hand came into contact with the titan Dina Fritz. Not just anyone can control titans as the power is 'locked' unless the possessor is of royal blood or is in contact with a person of royal blood. Also, not just anyone can inherit the 9 titan-shifters as only Subjects of Ymir can be turned to titans via injection and possess or inherit them. Even if a titan-shifter dies, they are inherited by another person born after the previous possessor who must also be a Subject of Ymir, and not just by some random person.
For your second question,
Can we conclude that there are other titan shifters who fight against humanity? This matters depending on the perspective. Based on that conversation, you can only conclude that there are other people which seem to have the same objective and goal as Zeke and the other honorary Marleyans sent to recover the Founding Titan. For Marleyans, they see Eldians as a threat to humanity because of the suffering they experience in the past under the Eldian empire, hence there desire to take away the titan-shifter powers from them. For Eldians, they see Marley as a threat to humanity because of how they keep on trying to kidnap Eren, their biggest hope in fighting back against the titans. Are they really fighting for humanity or are they just fighting for their own survival?