You can shoot skeet with a pump fine.
You can shoot skeet with an 18" but if you're new to it, won't hit much.
Shoot with your friend's shotgun for a while, if you like skeet, get a 28" spare barrel for your Mossberg. I've never had a problem hitting doubles with a pump.
In some cases, it's cheaper to get a spare shotgun, 28" barreled used Mossberg 500s are inexpensive and common used. I picked one up for under $200 with tax before. Needed some cleanup work, but it's a good gun.
"And Shepherds we shall be. For thee, my Lord for thee. Power hath descended forth from thy hand, that our feet may swiftly carry out thy command. So we shall flow a river forth to thee and teeming with souls shall it ever be. In nomine Patris, et fili, et Spiritus Sancti. Amen."