Bicycle Mechanics - OK to replace tension pulley with a smaller one?

I have had a devil of a time getting the correct replacement tension pulley for my Trek 820. The bike now has a Shimano Acera RD and the teeth on the tension pully are worn to nubs. I bought some replacement pulleys from the bike shop and they are smaller than what's on there - they have less teeth. Next I tried ordering a replacement pulley online, and I ordered a Shimano replacement pulley with the same number of teeth. Only problem is the hub (I think that's what it's called. It's the part of the pulley that the shaft goes through.) of the pulley I ordered is much wider than the original, which makes the replacement pulley too wide for the cage.
Can I use one of the smaller, less-teeth pulleys to replace the worn tension pulley? Seems like it wouldn't make much difference.

If it fits and spins smoothly, it'll work fine. I forget Shimano's marketing fluff for why they added teeth to the pulleys on some deraillers but I doubt it makes any real difference.

The lower (idler) pulley is very forgiving. As long as it's small enough to fit and has the same bolt hole diameter it'll work. There's also the issue of overall width and side offset, which has to match the upper, but most are very similar.

If it fits and spins smoothly, it'll work fine. I forget Shimano's marketing fluff for why they added teeth to the pulleys on some deraillers but I doubt it makes any real difference.

+1 As long as it spins freely and doesn't cause the chain to rub on the derailleur cage you should be OK. (It will reduce your derailleur's capacity a little.) I'm perplexed, though, as to why the Shimano replacement pulley doesn't fit. Unless I'm mistaken, Shimano has manufactured Acera derailleurs in both 7-speed and 8-speed. I'm not sure what the difference is, but it's possible, I suppose, that they have different width cages and pulleys. Jenson lists pulleys for both, so you might contact them and see if they can clear things up for you.

Edit: IIRC, the pulleys on the Acera are different sizes. The guide pulley is 13t and the tension pulley is 15t. Is it possible that they've gotten mixed up somehow?

11T pulleys last longer than 10T ones because u are always alternating teeth so in each revolution are not always the same Teeth the ones that are in contact with the chain, shimano road pullyes last forever anybody noticed that.

I'm surprised your bike shop couldn't order the correct ones! Here is a brand new Acera SGS RD for $18: http://www.outsideoutfitters.com/ps-16827-1049-shimano-acera-m360-7-8sp-rear-derailleur.aspx