Or you could just stick with a simple two-screw ligature that will hold the reed to the mouthpiece good, and practice.... Ligatures aren't going to make a world of a difference as long as it's doing its job and you have a good mouthpiece (Morgan being very good...). Congrats on getting a new mouthpiece :-)

so I have to buy the Olegature size 1?

If it's slimmer it might feel a little different...just as long as you take in enough mouthpiece (not many people seem too...and it opens up the sound a LOT)...

as far as olegature goes, either 1s and 2g will fit. 1s will definitely give more perfect, snug fit, while 2g will need to be pushed almost to the end...or, you can simply get a really thick mouthpiece patch to replace that crappy anti-slip cushion they give you...the stock cushion falled apart on me for many times...and you have to screw that thing all they way in....it is not the ideal "fit", but it works as an olegature should. then, i do think if you use thin reeds, 2g would not fit at all...only difference between 1s and 2g for me was that 2g sounds mellower, and 1s sounds more crisp. bonade clarinet ligatures are decent too, although the plates designed for clarinet reeds sometimes damaged my reed. if you wanna go really cheap, the cheap silver clarinet ligs, strings, rubber bands, and duct tapes work decent^^