[I bought this rose plant a couple of months ago. I recently noticed there's a smaller baby rose plant next to the main plant. I'm not sure if it's a new seedling or just an offshoot of the main plant. Can I remove it and repot the small one so that it can grow into its own plant or should I just leave it here I don't think there's space for it to grow big in this pot.
I’m afraid but that “baby” is something you want to get rid of, and soon. This is a shoot from the plant, not a seedling, but from the root stock, not from the grafted top.
Look at your stem and note the thicker part, from where the other stems originate. That’s the point where the plant material was introduced that is going to give you the kind of growth and flowers you bought your rose for. Everything below is a sucker and will develop into what the root stock would have grown into naturally. As the root stock was bred for sturdiness and its ability to supply the top, it won’t give you pretty flowers and even worse, the plant would direct its energy into this sucker, not the grafted bits.
When you remove it, it’s best to dig a bit until you can see where it come from and then pull it off. Cutting, especially too high, can encourage new suckers and the cycle repeats itself.
Planting the sucker is possible, but the results will probably not be very attractive. I wouldn’t bother.