The best and most stable boot to use when classical roller skiing is a combi boot. Combi boots are used for skating and classical technique because they are more ridged in the ankle area but yet flexible in the sole. The rigidity in the ankle area offers you more stability which is beneficial if you are just learning how to roller ski. Skating boots are getting stiffer and more ridged all the time which is great for the roller skiers. However, cross country ski boots and bindings are designed for cross country skis not roller skis.
Many people use their old boots and binding on their roller skis which is a big mistake.
Roller skis need good boots and bindings because you can form bad habits and are more likely to fall. If you want to simulate cross country skiing you must simulate the stability of the cross country ski. So, use the best boots and binding you can afford for roller skiing and you will not regret it. The majority of roller ski manufactures use cross country ski boots and bindings. However, there is one model out there that uses an in-line skate boot and a more ridged binding which is designed specifically for roller skiing.