Can a 7mo mini colt that hasn't dropped get a 10yo POA mare pregnant?

It is unusual for a 7 mth to have the "drive" but it is possible. If the testicles truly haven't dropped then again it is highly unlikely but still possible. Since he is only 7mths, his testicles may be too small to acutally see, you have to feel. They can and will actually float back and forth while they are growing in such a young colt. If he will let you, feel his scrotum and see if you can feel his testicles. The first question I have to ask clients that call this vet clinic is if both testicles are down, so most want to know before preparing for the surgery.

Yes he can breed to her and you should have the vet check and decide if he has "dropped". I had my colt gelded at 6 mo old, everyone was telling me that he hadnt dropped and the vet wouldn't be able to do it and with a mild sedative the vet was able to feel enough to find that they had actually dropped but just didn't stand out.

he is still too young 15/18 months would be about the earliest

as for the size difference ,Yes they will breed if she comes in heat she will kneel down for him its just natures way ,if there is a stallion and a mare they will breed of that you can be sure