rnjng,
you can change your hoses to 1 1/2" if you want too which would probably be the best thing to do to get the most flow with the least amount of resistance. Or you can get step down adapters from your local hardware store that step down from 1 1/2" to 1 1/4" and use the same hoses you already have. I don't know if it would make that big of a difference. Either way I believe you will be fine.

As far as the valve clamp goes, just make sure when you get the clamp snug that you tighten each side a little at a time alternating between the 2 screws, you can probably leave a little gap between the clamps, if it leaks you can always tighten a little bit more until it doesn't leak any more.