I find the 1 1/2G to be a bit big and instead use my regular DE Euph mouthpiece (but it's already pretty big). I find the Wick 3AL to be a bit small for this purpose. It depends partly on your euphonium and partly on you. Some experimentation (roughly in the range of a 3AL-1AL or 1.5G) should be expected -- and you should expect to change your mind as you gain more experience using the horn in that way. A good "exercise" is to go through the Tuba 1 parts in the Tuba Christmas book. I've used a euph several times on that part.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)