I'm 5'10 150lbs, does 2600 calories sound ok to slowly gain weight?
It depends on how much, ON AVERAGE, you tend to metabolize in a day. 2,600 calories is not an exorbitantly great amount of energy, and many males even of your bodyweight will regularly use 3,000 or more calories worth of energy. If a typical day for you involves sitting around a lot and not doing much exercise, then there's a good chance 2,600 would be enough to slowly gain some poundage.
But most importantly, you need to see what actually happens at a given regular ballpark energy consumption. Some people's responses to overeating are wildly different to others. For example, some bodies tend to react to hypercaloric situations by increasing spontaneous physical activity. These are the types of people who always seem to be busy fidgeting, maintaining posture, and moving around a lot. I'm one of those types, and have been in scenarios where even 4,500 calories wasn't putting on any weight at 190 lbs.
So in summary the answer is that
it depends. You might find 2,600 is plenty, or you may discover that 3,000 isn't quite enough. So take morning weight measurements regularly and track the general trends. Learn how your body reacts, and act accordingly.