I would not put two older females together. Been there, tried that. There is a reason they're called Bitches.
I'd say you would be better off with a male or younger female.
For me, I'll take a puppy over an older adult any day. Crate train it, give it plenty of Nylabones and Kong toys to chew on (if that is your Harlequin, you know what these are
), and it's not that big of a deal. Our newest pack member was a 6 month old Pembroke Welsh Corgi. Our Dane, 5 at the time, took to her immediately and the two of them still play constantly. Our male Corgi no longer plays with the Dane, but spends hours running around with the little female.
You may already know this but, there is no equality in a dogs mind. You're either higher on the ladder, or lower, but nobody's standing on the same rung. They will work it out and many times size and sex has little to do with it. As Jerry has said, don't try to make the new dog feel at home at the expense of your current pet. You will send a message of expected status in the pack and this could cause problems.