Yeah they should be fine, as far as I can tell there is no specific rating required for airsoft, but the closer to A the letter is the better the protection bearing in mind these are rated against industrial debris like metal rather than plastic
Although if your trying to stay cool by reducing layers, a hydration system does cover a large portion of your back and tend to make me sweat like a madman
I agree too, but if its not much of a difference I prefer to buy from a shop. Firstly to support UK Businesses and secondly all the shiney shiney stuff they have on the walls !
http://www.ebay.co.uk/itm/New-British-Army-Multicam-MTP-Yoke-Molle-Hip-Pad-and-Belt-Large-dpm-plce-/230980750935?pt=UK_Collectables_Militaria_LE&hash=item35c7868e57
That's genuine MTP though rather expensive